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/*
* Find the post-mortem offset from a core dump
*
* Copyright (C) 2026 Willy Tarreau <w@1wt.eu>
*
* Permission is hereby granted, free of charge, to any person obtaining
* a copy of this software and associated documentation files (the
* "Software"), to deal in the Software without restriction, including
* without limitation the rights to use, copy, modify, merge, publish,
* distribute, sublicense, and/or sell copies of the Software, and to
* permit persons to whom the Software is furnished to do so, subject to
* the following conditions:
*
* The above copyright notice and this permission notice shall be
* included in all copies or substantial portions of the Software.
*
* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
* EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES
* OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
* NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT
* HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY,
* WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
* FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
* OTHER DEALINGS IN THE SOFTWARE.
*/
/* Note: builds with no option under glibc, and can be built as a minimal
* uploadable static executable using nolibc as well:
gcc -o pm-from-core -nostdinc -nostdlib -s -Os -static -fno-ident \
-fno-exceptions -fno-asynchronous-unwind-tables -fno-unwind-tables \
-Wl,--gc-sections,--orphan-handling=discard,-znoseparate-code \
-I /path/to/nolibc-sysroot/include pm-from-core.c
*/
#define _GNU_SOURCE
#include <sys/mman.h>
#include <sys/stat.h>
#include <elf.h>
#include <fcntl.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#if defined(__GLIBC__)
# define my_memmem memmem
#else
void *my_memmem(const void *haystack, size_t haystacklen,
const void *needle, size_t needlelen)
{
while (haystacklen >= needlelen) {
if (!memcmp(haystack, needle, needlelen))
return (void*)haystack;
haystack++;
haystacklen--;
}
return NULL;
}
#endif
#define MAGIC "POST-MORTEM STARTS HERE+7654321\0"
int main(int argc, char **argv)
{
Elf64_Ehdr *ehdr;
Elf64_Phdr *phdr;
struct stat st;
uint8_t *mem;
int i, fd;
if (argc < 2) {
printf("Usage: %s <core_file>\n", argv[0]);
exit(1);
}
fd = open(argv[1], O_RDONLY);
/* Let's just map the core dump as an ELF header */
fstat(fd, &st);
mem = mmap(NULL, st.st_size, PROT_READ, MAP_PRIVATE, fd, 0);
if (mem == MAP_FAILED) {
perror("mmap()");
exit(1);
}
/* get the program headers */
ehdr = (Elf64_Ehdr *)mem;
/* check that it's really a core. Should be "\x7fELF" */
if (memcmp(ehdr->e_ident, ELFMAG, SELFMAG) != 0) {
fprintf(stderr, "ELF magic not found.\n");
exit(1);
}
if (ehdr->e_ident[EI_CLASS] != ELFCLASS64) {
fprintf(stderr, "Only 64-bit ELF supported.\n");
exit(1);
}
if (ehdr->e_type != ET_CORE) {
fprintf(stderr, "ELF type %d, not a core dump.\n", ehdr->e_type);
exit(1);
}
/* OK we can safely go with program headers */
phdr = (Elf64_Phdr *)(mem + ehdr->e_phoff);
for (i = 0; i < ehdr->e_phnum; i++) {
uint64_t size = phdr[i].p_filesz;
uint64_t offset = phdr[i].p_offset;
uint64_t vaddr = phdr[i].p_vaddr;
uint64_t found_ofs;
uint8_t *found;
if (phdr[i].p_type != PT_LOAD)
continue;
//printf("Scanning segment %d...\n", ehdr->e_phnum);
//printf("\r%-5d: off=%lx va=%lx sz=%lx ", i, (long)offset, (long)vaddr, (long)size);
if (!size)
continue;
if (size >= 1048576) // don't scan large segments
continue;
found = my_memmem(mem + offset, size, MAGIC, sizeof(MAGIC) - 1);
if (!found)
continue;
found_ofs = found - (mem + offset);
printf("Found post-mortem magic in segment %d:\n", i);
printf(" Core File Offset: 0x%lx (0x%lx + 0x%lx)\n", offset + found_ofs, offset, found_ofs);
printf(" Runtime VAddr: 0x%lx (0x%lx + 0x%lx)\n", vaddr + found_ofs, vaddr, found_ofs);
printf(" Segment Size: 0x%lx\n", size);
printf("\nIn gdb, copy-paste this line:\n\n pm_init 0x%lx\n\n", vaddr + found_ofs);
return 0;
}
//printf("\r%75s\n", "\r");
printf("post-mortem magic not found\n");
return 1;
}
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