ftrace: Add C version of recordmcount compile time code
John Reiser [Wed, 13 Oct 2010 19:12:54 +0000 (15:12 -0400)]
Currently, the mcount callers are found with a perl script that does
an objdump on every file in the kernel. This is a C version of that
same code which should increase the performance time of compiling
the kernel with dynamic ftrace enabled.

Signed-off-by: John Reiser <jreiser@bitwagon.com>

[ Updated the code to include .text.unlikely section as well as
  changing the format to follow Linux coding style. ]

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

scripts/recordmcount.c [new file with mode: 0644]

diff --git a/scripts/recordmcount.c b/scripts/recordmcount.c
new file mode 100644 (file)
index 0000000..34f32be
--- /dev/null
@@ -0,0 +1,885 @@
+/*
+ * recordmcount.c: construct a table of the locations of calls to 'mcount'
+ * so that ftrace can find them quickly.
+ * Copyright 2009 John F. Reiser <jreiser@BitWagon.com>.  All rights reserved.
+ * Licensed under the GNU General Public License, version 2 (GPLv2).
+ *
+ * Restructured to fit Linux format, as well as other updates:
+ *  Copyright 2010 Steven Rostedt <srostedt@redhat.com>, Red Hat Inc.
+ */
+
+/*
+ * Strategy: alter the .o file in-place.
+ *
+ * Append a new STRTAB that has the new section names, followed by a new array
+ * ElfXX_Shdr[] that has the new section headers, followed by the section
+ * contents for __mcount_loc and its relocations.  The old shstrtab strings,
+ * and the old ElfXX_Shdr[] array, remain as "garbage" (commonly, a couple
+ * kilobytes.)  Subsequent processing by /bin/ld (or the kernel module loader)
+ * will ignore the garbage regions, because they are not designated by the
+ * new .e_shoff nor the new ElfXX_Shdr[].  [In order to remove the garbage,
+ * then use "ld -r" to create a new file that omits the garbage.]
+ */
+
+#include <sys/types.h>
+#include <sys/mman.h>
+#include <sys/stat.h>
+#include <elf.h>
+#include <fcntl.h>
+#include <setjmp.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+
+static int fd_map;     /* File descriptor for file being modified. */
+static int mmap_failed; /* Boolean flag. */
+static void *ehdr_curr; /* current ElfXX_Ehdr *  for resource cleanup */
+static char gpfx;      /* prefix for global symbol name (sometimes '_') */
+static struct stat sb; /* Remember .st_size, etc. */
+static jmp_buf jmpenv; /* setjmp/longjmp per-file error escape */
+
+/* setjmp() return values */
+enum {
+       SJ_SETJMP = 0,  /* hardwired first return */
+       SJ_FAIL,
+       SJ_SUCCEED
+};
+
+/* Per-file resource cleanup when multiple files. */
+static void
+cleanup(void)
+{
+       if (!mmap_failed)
+               munmap(ehdr_curr, sb.st_size);
+       else
+               free(ehdr_curr);
+       close(fd_map);
+}
+
+static void __attribute__((noreturn))
+fail_file(void)
+{
+       cleanup();
+       longjmp(jmpenv, SJ_FAIL);
+}
+
+static void __attribute__((noreturn))
+succeed_file(void)
+{
+       cleanup();
+       longjmp(jmpenv, SJ_SUCCEED);
+}
+
+/* ulseek, uread, ...:  Check return value for errors. */
+
+static off_t
+ulseek(int const fd, off_t const offset, int const whence)
+{
+       off_t const w = lseek(fd, offset, whence);
+       if ((off_t)-1 == w) {
+               perror("lseek");
+               fail_file();
+       }
+       return w;
+}
+
+static size_t
+uread(int const fd, void *const buf, size_t const count)
+{
+       size_t const n = read(fd, buf, count);
+       if (n != count) {
+               perror("read");
+               fail_file();
+       }
+       return n;
+}
+
+static size_t
+uwrite(int const fd, void const *const buf, size_t const count)
+{
+       size_t const n = write(fd, buf, count);
+       if (n != count) {
+               perror("write");
+               fail_file();
+       }
+       return n;
+}
+
+static void *
+umalloc(size_t size)
+{
+       void *const addr = malloc(size);
+       if (0 == addr) {
+               fprintf(stderr, "malloc failed: %zu bytes\n", size);
+               fail_file();
+       }
+       return addr;
+}
+
+/*
+ * Get the whole file as a programming convenience in order to avoid
+ * malloc+lseek+read+free of many pieces.  If successful, then mmap
+ * avoids copying unused pieces; else just read the whole file.
+ * Open for both read and write; new info will be appended to the file.
+ * Use MAP_PRIVATE so that a few changes to the in-memory ElfXX_Ehdr
+ * do not propagate to the file until an explicit overwrite at the last.
+ * This preserves most aspects of consistency (all except .st_size)
+ * for simultaneous readers of the file while we are appending to it.
+ * However, multiple writers still are bad.  We choose not to use
+ * locking because it is expensive and the use case of kernel build
+ * makes multiple writers unlikely.
+ */
+static void *mmap_file(char const *fname)
+{
+       void *addr;
+
+       fd_map = open(fname, O_RDWR);
+       if (0 > fd_map || 0 > fstat(fd_map, &sb)) {
+               perror(fname);
+               fail_file();
+       }
+       if (!S_ISREG(sb.st_mode)) {
+               fprintf(stderr, "not a regular file: %s\n", fname);
+               fail_file();
+       }
+       addr = mmap(0, sb.st_size, PROT_READ|PROT_WRITE, MAP_PRIVATE,
+                   fd_map, 0);
+       mmap_failed = 0;
+       if (MAP_FAILED == addr) {
+               mmap_failed = 1;
+               addr = umalloc(sb.st_size);
+               uread(fd_map, addr, sb.st_size);
+       }
+       return addr;
+}
+
+/* w8rev, w8nat, ...: Handle endianness. */
+
+static uint64_t w8rev(uint64_t const x)
+{
+       return   ((0xff & (x >> (0 * 8))) << (7 * 8))
+              | ((0xff & (x >> (1 * 8))) << (6 * 8))
+              | ((0xff & (x >> (2 * 8))) << (5 * 8))
+              | ((0xff & (x >> (3 * 8))) << (4 * 8))
+              | ((0xff & (x >> (4 * 8))) << (3 * 8))
+              | ((0xff & (x >> (5 * 8))) << (2 * 8))
+              | ((0xff & (x >> (6 * 8))) << (1 * 8))
+              | ((0xff & (x >> (7 * 8))) << (0 * 8));
+}
+
+static uint32_t w4rev(uint32_t const x)
+{
+       return   ((0xff & (x >> (0 * 8))) << (3 * 8))
+              | ((0xff & (x >> (1 * 8))) << (2 * 8))
+              | ((0xff & (x >> (2 * 8))) << (1 * 8))
+              | ((0xff & (x >> (3 * 8))) << (0 * 8));
+}
+
+static uint32_t w2rev(uint16_t const x)
+{
+       return   ((0xff & (x >> (0 * 8))) << (1 * 8))
+              | ((0xff & (x >> (1 * 8))) << (0 * 8));
+}
+
+static uint64_t w8nat(uint64_t const x)
+{
+       return x;
+}
+
+static uint32_t w4nat(uint32_t const x)
+{
+       return x;
+}
+
+static uint32_t w2nat(uint16_t const x)
+{
+       return x;
+}
+
+static uint64_t (*w8)(uint64_t);
+static uint32_t (*w)(uint32_t);
+static uint32_t (*w2)(uint16_t);
+
+/* Names of the sections that could contain calls to mcount. */
+static int
+is_mcounted_section_name(char const *const txtname)
+{
+       return 0 == strcmp(".text",          txtname) ||
+               0 == strcmp(".sched.text",    txtname) ||
+               0 == strcmp(".spinlock.text", txtname) ||
+               0 == strcmp(".irqentry.text", txtname) ||
+               0 == strcmp(".text.unlikely", txtname);
+}
+
+/* Append the new shstrtab, Elf32_Shdr[], __mcount_loc and its relocations. */
+static void append32(Elf32_Ehdr *const ehdr,
+                    Elf32_Shdr *const shstr,
+                    uint32_t const *const mloc0,
+                    uint32_t const *const mlocp,
+                    Elf32_Rel const *const mrel0,
+                    Elf32_Rel const *const mrelp,
+                    unsigned int const rel_entsize,
+                    unsigned int const symsec_sh_link)
+{
+       /* Begin constructing output file */
+       Elf32_Shdr mcsec;
+       char const *mc_name = (sizeof(Elf32_Rela) == rel_entsize)
+               ? ".rela__mcount_loc"
+               :  ".rel__mcount_loc";
+       unsigned const old_shnum = w2(ehdr->e_shnum);
+       uint32_t const old_shoff = w(ehdr->e_shoff);
+       uint32_t const old_shstr_sh_size   = w(shstr->sh_size);
+       uint32_t const old_shstr_sh_offset = w(shstr->sh_offset);
+       uint32_t t = 1 + strlen(mc_name) + w(shstr->sh_size);
+       uint32_t new_e_shoff;
+
+       shstr->sh_size = w(t);
+       shstr->sh_offset = w(sb.st_size);
+       t += sb.st_size;
+       t += (3u & -t);  /* 4-byte align */
+       new_e_shoff = t;
+
+       /* body for new shstrtab */
+       ulseek(fd_map, sb.st_size, SEEK_SET);
+       uwrite(fd_map, old_shstr_sh_offset + (void *)ehdr, old_shstr_sh_size);
+       uwrite(fd_map, mc_name, 1 + strlen(mc_name));
+
+       /* old(modified) Elf32_Shdr table, 4-byte aligned */
+       ulseek(fd_map, t, SEEK_SET);
+       t += sizeof(Elf32_Shdr) * old_shnum;
+       uwrite(fd_map, old_shoff + (void *)ehdr,
+              sizeof(Elf32_Shdr) * old_shnum);
+
+       /* new sections __mcount_loc and .rel__mcount_loc */
+       t += 2*sizeof(mcsec);
+       mcsec.sh_name = w((sizeof(Elf32_Rela) == rel_entsize) + strlen(".rel")
+               + old_shstr_sh_size);
+       mcsec.sh_type = w(SHT_PROGBITS);
+       mcsec.sh_flags = w(SHF_ALLOC);
+       mcsec.sh_addr = 0;
+       mcsec.sh_offset = w(t);
+       mcsec.sh_size = w((void *)mlocp - (void *)mloc0);
+       mcsec.sh_link = 0;
+       mcsec.sh_info = 0;
+       mcsec.sh_addralign = w(4);
+       mcsec.sh_entsize = w(4);
+       uwrite(fd_map, &mcsec, sizeof(mcsec));
+
+       mcsec.sh_name = w(old_shstr_sh_size);
+       mcsec.sh_type = (sizeof(Elf32_Rela) == rel_entsize)
+               ? w(SHT_RELA)
+               : w(SHT_REL);
+       mcsec.sh_flags = 0;
+       mcsec.sh_addr = 0;
+       mcsec.sh_offset = w((void *)mlocp - (void *)mloc0 + t);
+       mcsec.sh_size   = w((void *)mrelp - (void *)mrel0);
+       mcsec.sh_link = w(symsec_sh_link);
+       mcsec.sh_info = w(old_shnum);
+       mcsec.sh_addralign = w(4);
+       mcsec.sh_entsize = w(rel_entsize);
+       uwrite(fd_map, &mcsec, sizeof(mcsec));
+
+       uwrite(fd_map, mloc0, (void *)mlocp - (void *)mloc0);
+       uwrite(fd_map, mrel0, (void *)mrelp - (void *)mrel0);
+
+       ehdr->e_shoff = w(new_e_shoff);
+       ehdr->e_shnum = w2(2 + w2(ehdr->e_shnum));  /* {.rel,}__mcount_loc */
+       ulseek(fd_map, 0, SEEK_SET);
+       uwrite(fd_map, ehdr, sizeof(*ehdr));
+}
+
+/*
+ * append64 and append32 (and other analogous pairs) could be templated
+ * using C++, but the complexity is high.  (For an example, look at p_elf.h
+ * in the source for UPX, http://upx.sourceforge.net)  So: remember to make
+ * the corresponding change in the routine for the other size.
+ */
+static void append64(Elf64_Ehdr *const ehdr,
+                    Elf64_Shdr *const shstr,
+                    uint64_t const *const mloc0,
+                    uint64_t const *const mlocp,
+                    Elf64_Rel const *const mrel0,
+                    Elf64_Rel const *const mrelp,
+                    unsigned int const rel_entsize,
+                    unsigned int const symsec_sh_link)
+{
+       /* Begin constructing output file */
+       Elf64_Shdr mcsec;
+       char const *mc_name = (sizeof(Elf64_Rela) == rel_entsize)
+               ? ".rela__mcount_loc"
+               :  ".rel__mcount_loc";
+       unsigned const old_shnum = w2(ehdr->e_shnum);
+       uint64_t const old_shoff = w8(ehdr->e_shoff);
+       uint64_t const old_shstr_sh_size   = w8(shstr->sh_size);
+       uint64_t const old_shstr_sh_offset = w8(shstr->sh_offset);
+       uint64_t t = 1 + strlen(mc_name) + w8(shstr->sh_size);
+       uint64_t new_e_shoff;
+
+       shstr->sh_size = w8(t);
+       shstr->sh_offset = w8(sb.st_size);
+       t += sb.st_size;
+       t += (7u & -t);  /* 8-byte align */
+       new_e_shoff = t;
+
+       /* body for new shstrtab */
+       ulseek(fd_map, sb.st_size, SEEK_SET);
+       uwrite(fd_map, old_shstr_sh_offset + (void *)ehdr, old_shstr_sh_size);
+       uwrite(fd_map, mc_name, 1 + strlen(mc_name));
+
+       /* old(modified) Elf64_Shdr table, 8-byte aligned */
+       ulseek(fd_map, t, SEEK_SET);
+       t += sizeof(Elf64_Shdr) * old_shnum;
+       uwrite(fd_map, old_shoff + (void *)ehdr,
+               sizeof(Elf64_Shdr) * old_shnum);
+
+       /* new sections __mcount_loc and .rel__mcount_loc */
+       t += 2*sizeof(mcsec);
+       mcsec.sh_name = w((sizeof(Elf64_Rela) == rel_entsize) + strlen(".rel")
+               + old_shstr_sh_size);
+       mcsec.sh_type = w(SHT_PROGBITS);
+       mcsec.sh_flags = w8(SHF_ALLOC);
+       mcsec.sh_addr = 0;
+       mcsec.sh_offset = w8(t);
+       mcsec.sh_size = w8((void *)mlocp - (void *)mloc0);
+       mcsec.sh_link = 0;
+       mcsec.sh_info = 0;
+       mcsec.sh_addralign = w8(8);
+       mcsec.sh_entsize = w8(8);
+       uwrite(fd_map, &mcsec, sizeof(mcsec));
+
+       mcsec.sh_name = w(old_shstr_sh_size);
+       mcsec.sh_type = (sizeof(Elf64_Rela) == rel_entsize)
+               ? w(SHT_RELA)
+               : w(SHT_REL);
+       mcsec.sh_flags = 0;
+       mcsec.sh_addr = 0;
+       mcsec.sh_offset = w8((void *)mlocp - (void *)mloc0 + t);
+       mcsec.sh_size   = w8((void *)mrelp - (void *)mrel0);
+       mcsec.sh_link = w(symsec_sh_link);
+       mcsec.sh_info = w(old_shnum);
+       mcsec.sh_addralign = w8(8);
+       mcsec.sh_entsize = w8(rel_entsize);
+       uwrite(fd_map, &mcsec, sizeof(mcsec));
+
+       uwrite(fd_map, mloc0, (void *)mlocp - (void *)mloc0);
+       uwrite(fd_map, mrel0, (void *)mrelp - (void *)mrel0);
+
+       ehdr->e_shoff = w8(new_e_shoff);
+       ehdr->e_shnum = w2(2 + w2(ehdr->e_shnum));  /* {.rel,}__mcount_loc */
+       ulseek(fd_map, 0, SEEK_SET);
+       uwrite(fd_map, ehdr, sizeof(*ehdr));
+}
+
+/*
+ * Look at the relocations in order to find the calls to mcount.
+ * Accumulate the section offsets that are found, and their relocation info,
+ * onto the end of the existing arrays.
+ */
+static uint32_t *sift32_rel_mcount(uint32_t *mlocp,
+                                  unsigned const offbase,
+                                  Elf32_Rel **const mrelpp,
+                                  Elf32_Shdr const *const relhdr,
+                                  Elf32_Ehdr const *const ehdr,
+                                  unsigned const recsym,
+                                  uint32_t const recval,
+                                  unsigned const reltype)
+{
+       uint32_t *const mloc0 = mlocp;
+       Elf32_Rel *mrelp = *mrelpp;
+       Elf32_Shdr *const shdr0 = (Elf32_Shdr *)(w(ehdr->e_shoff)
+               + (void *)ehdr);
+       unsigned const symsec_sh_link = w(relhdr->sh_link);
+       Elf32_Shdr const *const symsec = &shdr0[symsec_sh_link];
+       Elf32_Sym const *const sym0 = (Elf32_Sym const *)(w(symsec->sh_offset)
+               + (void *)ehdr);
+
+       Elf32_Shdr const *const strsec = &shdr0[w(symsec->sh_link)];
+       char const *const str0 = (char const *)(w(strsec->sh_offset)
+               + (void *)ehdr);
+
+       Elf32_Rel const *const rel0 = (Elf32_Rel const *)(w(relhdr->sh_offset)
+               + (void *)ehdr);
+       unsigned rel_entsize = w(relhdr->sh_entsize);
+       unsigned const nrel = w(relhdr->sh_size) / rel_entsize;
+       Elf32_Rel const *relp = rel0;
+
+       unsigned mcountsym = 0;
+       unsigned t;
+
+       for (t = nrel; t; --t) {
+               if (!mcountsym) {
+                       Elf32_Sym const *const symp =
+                               &sym0[ELF32_R_SYM(w(relp->r_info))];
+
+                       if (0 == strcmp((('_' == gpfx) ? "_mcount" : "mcount"),
+                                       &str0[w(symp->st_name)]))
+                               mcountsym = ELF32_R_SYM(w(relp->r_info));
+               }
+               if (mcountsym == ELF32_R_SYM(w(relp->r_info))) {
+                       uint32_t const addend = w(w(relp->r_offset) - recval);
+                       mrelp->r_offset = w(offbase
+                               + ((void *)mlocp - (void *)mloc0));
+                       mrelp->r_info = w(ELF32_R_INFO(recsym, reltype));
+                       if (sizeof(Elf32_Rela) == rel_entsize) {
+                               ((Elf32_Rela *)mrelp)->r_addend = addend;
+                               *mlocp++ = 0;
+                       } else
+                               *mlocp++ = addend;
+
+                       mrelp = (Elf32_Rel *)(rel_entsize + (void *)mrelp);
+               }
+               relp = (Elf32_Rel const *)(rel_entsize + (void *)relp);
+       }
+       *mrelpp = mrelp;
+       return mlocp;
+}
+
+static uint64_t *sift64_rel_mcount(uint64_t *mlocp,
+                                  unsigned const offbase,
+                                  Elf64_Rel **const mrelpp,
+                                  Elf64_Shdr const *const relhdr,
+                                  Elf64_Ehdr const *const ehdr,
+                                  unsigned const recsym,
+                                  uint64_t const recval,
+                                  unsigned const reltype)
+{
+       uint64_t *const mloc0 = mlocp;
+       Elf64_Rel *mrelp = *mrelpp;
+       Elf64_Shdr *const shdr0 = (Elf64_Shdr *)(w8(ehdr->e_shoff)
+               + (void *)ehdr);
+       unsigned const symsec_sh_link = w(relhdr->sh_link);
+       Elf64_Shdr const *const symsec = &shdr0[symsec_sh_link];
+       Elf64_Sym const *const sym0 = (Elf64_Sym const *)(w8(symsec->sh_offset)
+               + (void *)ehdr);
+
+       Elf64_Shdr const *const strsec = &shdr0[w(symsec->sh_link)];
+       char const *const str0 = (char const *)(w8(strsec->sh_offset)
+               + (void *)ehdr);
+
+       Elf64_Rel const *const rel0 = (Elf64_Rel const *)(w8(relhdr->sh_offset)
+               + (void *)ehdr);
+       unsigned rel_entsize = w8(relhdr->sh_entsize);
+       unsigned const nrel = w8(relhdr->sh_size) / rel_entsize;
+       Elf64_Rel const *relp = rel0;
+
+       unsigned mcountsym = 0;
+       unsigned t;
+
+       for (t = nrel; 0 != t; --t) {
+               if (!mcountsym) {
+                       Elf64_Sym const *const symp =
+                               &sym0[ELF64_R_SYM(w8(relp->r_info))];
+                       char const *symname = &str0[w(symp->st_name)];
+
+                       if ('.' == symname[0])
+                               ++symname;  /* ppc64 hack */
+                       if (0 == strcmp((('_' == gpfx) ? "_mcount" : "mcount"),
+                                       symname))
+                               mcountsym = ELF64_R_SYM(w8(relp->r_info));
+               }
+
+               if (mcountsym == ELF64_R_SYM(w8(relp->r_info))) {
+                       uint64_t const addend = w8(w8(relp->r_offset) - recval);
+
+                       mrelp->r_offset = w8(offbase
+                               + ((void *)mlocp - (void *)mloc0));
+                       mrelp->r_info = w8(ELF64_R_INFO(recsym, reltype));
+                       if (sizeof(Elf64_Rela) == rel_entsize) {
+                               ((Elf64_Rela *)mrelp)->r_addend = addend;
+                               *mlocp++ = 0;
+                       } else
+                               *mlocp++ = addend;
+
+                       mrelp = (Elf64_Rel *)(rel_entsize + (void *)mrelp);
+               }
+               relp = (Elf64_Rel const *)(rel_entsize + (void *)relp);
+       }
+       *mrelpp = mrelp;
+
+       return mlocp;
+}
+
+/*
+ * Find a symbol in the given section, to be used as the base for relocating
+ * the table of offsets of calls to mcount.  A local or global symbol suffices,
+ * but avoid a Weak symbol because it may be overridden; the change in value
+ * would invalidate the relocations of the offsets of the calls to mcount.
+ * Often the found symbol will be the unnamed local symbol generated by
+ * GNU 'as' for the start of each section.  For example:
+ *    Num:    Value  Size Type    Bind   Vis      Ndx Name
+ *      2: 00000000     0 SECTION LOCAL  DEFAULT    1
+ */
+static unsigned find32_secsym_ndx(unsigned const txtndx,
+                                 char const *const txtname,
+                                 uint32_t *const recvalp,
+                                 Elf32_Shdr const *const symhdr,
+                                 Elf32_Ehdr const *const ehdr)
+{
+       Elf32_Sym const *const sym0 = (Elf32_Sym const *)(w(symhdr->sh_offset)
+               + (void *)ehdr);
+       unsigned const nsym = w(symhdr->sh_size) / w(symhdr->sh_entsize);
+       Elf32_Sym const *symp;
+       unsigned t;
+
+       for (symp = sym0, t = nsym; t; --t, ++symp) {
+               unsigned int const st_bind = ELF32_ST_BIND(symp->st_info);
+
+               if (txtndx == w2(symp->st_shndx)
+                       /* avoid STB_WEAK */
+                   && (STB_LOCAL == st_bind || STB_GLOBAL == st_bind)) {
+                       *recvalp = w(symp->st_value);
+                       return symp - sym0;
+               }
+       }
+       fprintf(stderr, "Cannot find symbol for section %d: %s.\n",
+               txtndx, txtname);
+       fail_file();
+}
+
+static unsigned find64_secsym_ndx(unsigned const txtndx,
+                                 char const *const txtname,
+                                 uint64_t *const recvalp,
+                                 Elf64_Shdr const *const symhdr,
+                                 Elf64_Ehdr const *const ehdr)
+{
+       Elf64_Sym const *const sym0 = (Elf64_Sym const *)(w8(symhdr->sh_offset)
+               + (void *)ehdr);
+       unsigned const nsym = w8(symhdr->sh_size) / w8(symhdr->sh_entsize);
+       Elf64_Sym const *symp;
+       unsigned t;
+
+       for (symp = sym0, t = nsym; t; --t, ++symp) {
+               unsigned int const st_bind = ELF64_ST_BIND(symp->st_info);
+
+               if (txtndx == w2(symp->st_shndx)
+                       /* avoid STB_WEAK */
+                   && (STB_LOCAL == st_bind || STB_GLOBAL == st_bind)) {
+                       *recvalp = w8(symp->st_value);
+                       return symp - sym0;
+               }
+       }
+       fprintf(stderr, "Cannot find symbol for section %d: %s.\n",
+               txtndx, txtname);
+       fail_file();
+}
+
+/*
+ * Evade ISO C restriction: no declaration after statement in
+ * has32_rel_mcount.
+ */
+static char const *
+__has32_rel_mcount(Elf32_Shdr const *const relhdr,  /* is SHT_REL or SHT_RELA */
+                  Elf32_Shdr const *const shdr0,
+                  char const *const shstrtab,
+                  char const *const fname)
+{
+       /* .sh_info depends on .sh_type == SHT_REL[,A] */
+       Elf32_Shdr const *const txthdr = &shdr0[w(relhdr->sh_info)];
+       char const *const txtname = &shstrtab[w(txthdr->sh_name)];
+
+       if (0 == strcmp("__mcount_loc", txtname)) {
+               fprintf(stderr, "warning: __mcount_loc already exists: %s\n",
+                       fname);
+               succeed_file();
+       }
+       if (SHT_PROGBITS != w(txthdr->sh_type) ||
+           !is_mcounted_section_name(txtname))
+               return NULL;
+       return txtname;
+}
+
+static char const *has32_rel_mcount(Elf32_Shdr const *const relhdr,
+                                   Elf32_Shdr const *const shdr0,
+                                   char const *const shstrtab,
+                                   char const *const fname)
+{
+       if (SHT_REL  != w(relhdr->sh_type) && SHT_RELA != w(relhdr->sh_type))
+               return NULL;
+       return __has32_rel_mcount(relhdr, shdr0, shstrtab, fname);
+}
+
+static char const *__has64_rel_mcount(Elf64_Shdr const *const relhdr,
+                                     Elf64_Shdr const *const shdr0,
+                                     char const *const shstrtab,
+                                     char const *const fname)
+{
+       /* .sh_info depends on .sh_type == SHT_REL[,A] */
+       Elf64_Shdr const *const txthdr = &shdr0[w(relhdr->sh_info)];
+       char const *const txtname = &shstrtab[w(txthdr->sh_name)];
+
+       if (0 == strcmp("__mcount_loc", txtname)) {
+               fprintf(stderr, "warning: __mcount_loc already exists: %s\n",
+                       fname);
+               succeed_file();
+       }
+       if (SHT_PROGBITS != w(txthdr->sh_type) ||
+           !is_mcounted_section_name(txtname))
+               return NULL;
+       return txtname;
+}
+
+static char const *has64_rel_mcount(Elf64_Shdr const *const relhdr,
+                                   Elf64_Shdr const *const shdr0,
+                                   char const *const shstrtab,
+                                   char const *const fname)
+{
+       if (SHT_REL  != w(relhdr->sh_type) && SHT_RELA != w(relhdr->sh_type))
+               return NULL;
+       return __has64_rel_mcount(relhdr, shdr0, shstrtab, fname);
+}
+
+static unsigned tot32_relsize(Elf32_Shdr const *const shdr0,
+                             unsigned nhdr,
+                             const char *const shstrtab,
+                             const char *const fname)
+{
+       unsigned totrelsz = 0;
+       Elf32_Shdr const *shdrp = shdr0;
+       for (; 0 != nhdr; --nhdr, ++shdrp) {
+               if (has32_rel_mcount(shdrp, shdr0, shstrtab, fname))
+                       totrelsz += w(shdrp->sh_size);
+       }
+       return totrelsz;
+}
+
+static unsigned tot64_relsize(Elf64_Shdr const *const shdr0,
+                             unsigned nhdr,
+                             const char *const shstrtab,
+                             const char *const fname)
+{
+       unsigned totrelsz = 0;
+       Elf64_Shdr const *shdrp = shdr0;
+
+       for (; nhdr; --nhdr, ++shdrp) {
+               if (has64_rel_mcount(shdrp, shdr0, shstrtab, fname))
+                       totrelsz += w8(shdrp->sh_size);
+       }
+       return totrelsz;
+}
+
+/* Overall supervision for Elf32 ET_REL file. */
+static void
+do32(Elf32_Ehdr *const ehdr, char const *const fname, unsigned const reltype)
+{
+       Elf32_Shdr *const shdr0 = (Elf32_Shdr *)(w(ehdr->e_shoff)
+               + (void *)ehdr);
+       unsigned const nhdr = w2(ehdr->e_shnum);
+       Elf32_Shdr *const shstr = &shdr0[w2(ehdr->e_shstrndx)];
+       char const *const shstrtab = (char const *)(w(shstr->sh_offset)
+               + (void *)ehdr);
+
+       Elf32_Shdr const *relhdr;
+       unsigned k;
+
+       /* Upper bound on space: assume all relevant relocs are for mcount. */
+       unsigned const totrelsz = tot32_relsize(shdr0, nhdr, shstrtab, fname);
+       Elf32_Rel *const mrel0 = umalloc(totrelsz);
+       Elf32_Rel *      mrelp = mrel0;
+
+       /* 2*sizeof(address) <= sizeof(Elf32_Rel) */
+       uint32_t *const mloc0 = umalloc(totrelsz>>1);
+       uint32_t *      mlocp = mloc0;
+
+       unsigned rel_entsize = 0;
+       unsigned symsec_sh_link = 0;
+
+       for (relhdr = shdr0, k = nhdr; k; --k, ++relhdr) {
+               char const *const txtname = has32_rel_mcount(relhdr, shdr0,
+                       shstrtab, fname);
+               if (txtname) {
+                       uint32_t recval = 0;
+                       unsigned const recsym = find32_secsym_ndx(
+                               w(relhdr->sh_info), txtname, &recval,
+                               &shdr0[symsec_sh_link = w(relhdr->sh_link)],
+                               ehdr);
+
+                       rel_entsize = w(relhdr->sh_entsize);
+                       mlocp = sift32_rel_mcount(mlocp,
+                               (void *)mlocp - (void *)mloc0, &mrelp,
+                               relhdr, ehdr, recsym, recval, reltype);
+               }
+       }
+       if (mloc0 != mlocp) {
+               append32(ehdr, shstr, mloc0, mlocp, mrel0, mrelp,
+                       rel_entsize, symsec_sh_link);
+       }
+       free(mrel0);
+       free(mloc0);
+}
+
+static void
+do64(Elf64_Ehdr *const ehdr, char const *const fname, unsigned const reltype)
+{
+       Elf64_Shdr *const shdr0 = (Elf64_Shdr *)(w8(ehdr->e_shoff)
+               + (void *)ehdr);
+       unsigned const nhdr = w2(ehdr->e_shnum);
+       Elf64_Shdr *const shstr = &shdr0[w2(ehdr->e_shstrndx)];
+       char const *const shstrtab = (char const *)(w8(shstr->sh_offset)
+               + (void *)ehdr);
+
+       Elf64_Shdr const *relhdr;
+       unsigned k;
+
+       /* Upper bound on space: assume all relevant relocs are for mcount. */
+       unsigned const totrelsz = tot64_relsize(shdr0, nhdr, shstrtab, fname);
+       Elf64_Rel *const mrel0 = umalloc(totrelsz);
+       Elf64_Rel *      mrelp = mrel0;
+
+       /* 2*sizeof(address) <= sizeof(Elf64_Rel) */
+       uint64_t *const mloc0 = umalloc(totrelsz>>1);
+       uint64_t *      mlocp = mloc0;
+
+       unsigned rel_entsize = 0;
+       unsigned symsec_sh_link = 0;
+
+       for ((relhdr = shdr0), k = nhdr; k; --k, ++relhdr) {
+               char const *const txtname = has64_rel_mcount(relhdr, shdr0,
+                       shstrtab, fname);
+               if (txtname) {
+                       uint64_t recval = 0;
+                       unsigned const recsym = find64_secsym_ndx(
+                               w(relhdr->sh_info), txtname, &recval,
+                               &shdr0[symsec_sh_link = w(relhdr->sh_link)],
+                               ehdr);
+
+                       rel_entsize = w8(relhdr->sh_entsize);
+                       mlocp = sift64_rel_mcount(mlocp,
+                               (void *)mlocp - (void *)mloc0, &mrelp,
+                               relhdr, ehdr, recsym, recval, reltype);
+               }
+       }
+       if (mloc0 != mlocp) {
+               append64(ehdr, shstr, mloc0, mlocp, mrel0, mrelp,
+                       rel_entsize, symsec_sh_link);
+       }
+       free(mrel0);
+       free(mloc0);
+}
+
+static void
+do_file(char const *const fname)
+{
+       Elf32_Ehdr *const ehdr = mmap_file(fname);
+       unsigned int reltype = 0;
+
+       ehdr_curr = ehdr;
+       w = w4nat;
+       w2 = w2nat;
+       w8 = w8nat;
+       switch (ehdr->e_ident[EI_DATA]) {
+               static unsigned int const endian = 1;
+       default: {
+               fprintf(stderr, "unrecognized ELF data encoding %d: %s\n",
+                       ehdr->e_ident[EI_DATA], fname);
+               fail_file();
+       } break;
+       case ELFDATA2LSB: {
+               if (1 != *(unsigned char const *)&endian) {
+                       /* main() is big endian, file.o is little endian. */
+                       w = w4rev;
+                       w2 = w2rev;
+                       w8 = w8rev;
+               }
+       } break;
+       case ELFDATA2MSB: {
+               if (0 != *(unsigned char const *)&endian) {
+                       /* main() is little endian, file.o is big endian. */
+                       w = w4rev;
+                       w2 = w2rev;
+                       w8 = w8rev;
+               }
+       } break;
+       }  /* end switch */
+       if (0 != memcmp(ELFMAG, ehdr->e_ident, SELFMAG)
+       ||  ET_REL != w2(ehdr->e_type)
+       ||  EV_CURRENT != ehdr->e_ident[EI_VERSION]) {
+               fprintf(stderr, "unrecognized ET_REL file %s\n", fname);
+               fail_file();
+       }
+
+       gpfx = 0;
+       switch (w2(ehdr->e_machine)) {
+       default: {
+               fprintf(stderr, "unrecognized e_machine %d %s\n",
+                       w2(ehdr->e_machine), fname);
+               fail_file();
+       } break;
+       case EM_386:     reltype = R_386_32;                   break;
+       case EM_ARM:     reltype = R_ARM_ABS32;                break;
+       case EM_IA_64:   reltype = R_IA64_IMM64;   gpfx = '_'; break;
+       case EM_PPC:     reltype = R_PPC_ADDR32;   gpfx = '_'; break;
+       case EM_PPC64:   reltype = R_PPC64_ADDR64; gpfx = '_'; break;
+       case EM_S390:    /* reltype: e_class    */ gpfx = '_'; break;
+       case EM_SH:      reltype = R_SH_DIR32;                 break;
+       case EM_SPARCV9: reltype = R_SPARC_64;     gpfx = '_'; break;
+       case EM_X86_64:  reltype = R_X86_64_64;                break;
+       }  /* end switch */
+
+       switch (ehdr->e_ident[EI_CLASS]) {
+       default: {
+               fprintf(stderr, "unrecognized ELF class %d %s\n",
+                       ehdr->e_ident[EI_CLASS], fname);
+               fail_file();
+       } break;
+       case ELFCLASS32: {
+               if (sizeof(Elf32_Ehdr) != w2(ehdr->e_ehsize)
+               ||  sizeof(Elf32_Shdr) != w2(ehdr->e_shentsize)) {
+                       fprintf(stderr,
+                               "unrecognized ET_REL file: %s\n", fname);
+                       fail_file();
+               }
+               if (EM_S390 == w2(ehdr->e_machine))
+                       reltype = R_390_32;
+               do32(ehdr, fname, reltype);
+       } break;
+       case ELFCLASS64: {
+               Elf64_Ehdr *const ghdr = (Elf64_Ehdr *)ehdr;
+               if (sizeof(Elf64_Ehdr) != w2(ghdr->e_ehsize)
+               ||  sizeof(Elf64_Shdr) != w2(ghdr->e_shentsize)) {
+                       fprintf(stderr,
+                               "unrecognized ET_REL file: %s\n", fname);
+                       fail_file();
+               }
+               if (EM_S390 == w2(ghdr->e_machine))
+                       reltype = R_390_64;
+               do64(ghdr, fname, reltype);
+       } break;
+       }  /* end switch */
+
+       cleanup();
+}
+
+int
+main(int argc, char const *argv[])
+{
+       int n_error = 0;  /* gcc-4.3.0 false positive complaint */
+       if (argc <= 1)
+               fprintf(stderr, "usage: recordmcount file.o...\n");
+       else  /* Process each file in turn, allowing deep failure. */
+       for (--argc, ++argv; 0 < argc; --argc, ++argv) {
+               int const sjval = setjmp(jmpenv);
+               switch (sjval) {
+               default: {
+                       fprintf(stderr, "internal error: %s\n", argv[0]);
+                       exit(1);
+               } break;
+               case SJ_SETJMP: {  /* normal sequence */
+                       /* Avoid problems if early cleanup() */
+                       fd_map = -1;
+                       ehdr_curr = NULL;
+                       mmap_failed = 1;
+                       do_file(argv[0]);
+               } break;
+               case SJ_FAIL: {  /* error in do_file or below */
+                       ++n_error;
+               } break;
+               case SJ_SUCCEED: {  /* premature success */
+                       /* do nothing */
+               } break;
+               }  /* end switch */
+       }
+       return !!n_error;
+}
+
+