DEV: gdb: add a utility to find the post-mortem address from a core

More and more often, core dumps retrieved on systems that build with
-fPIE by default are becoming unexploitable. Even functions and global
symbols get relocated and gdb cannot figure their final position.
Ironically the post_mortem struct lying in its own section that was
meant to ease its finding is not exempt from this problem.

The only remaining way is to inspect the core to search for the
post-mortem magic, figure its offset from the file and look up the
corresponding virtual address with objdump. This is quite a hassle.

This patch implements a simple utility that opens a 64-bit core dump,
scans the program headers looking for a data segment which contains
the post-mortem magic, and prints it on stdout. It also places the
"pm_init" command alone on its own line to ease copy-pasting into the
gdb console. With this, at least the other commands in this directory
work again and allow to inspect the program's state. E.g:

  $ ./getpm core.57612
  Found post-mortem magic in segment 5:
    Core File Offset: 0xfc600 (0xd5000 + 0x27600)
    Runtime VAddr:    0x5613e52b6600 (0x5613e528f000 + 0x27600)
    Segment Size:     0x28000

  In gdb, copy-paste this line:

     pm_init 0x5613e52b6600

It's worth noting that the program has so few dependencies that it even
builds with nolibc, allowing to upload a static executable into containers
being debugged and lacking development tools and compilers. The build
procedure is indicated inthe source code.

diff --git a/Makefile b/Makefile
index 006875048c4eacfa9e58a3a30966be51eec03b57..508bd75d72f8b927ec808fed85a5c50c88888dc0 100644 (file)
--- a/Makefile
+++ b/Makefile
@@ -1040,7 +1040,7 @@ IGNORE_OPTS=help install install-man install-doc install-bin \
        uninstall clean tags cscope tar git-tar version update-version \
        opts reg-tests reg-tests-help unit-tests admin/halog/halog dev/flags/flags \
        dev/haring/haring dev/ncpu/ncpu dev/poll/poll dev/tcploop/tcploop \
-       dev/term_events/term_events
+       dev/term_events/term_events dev/gdb/pm-from-core
 
 ifneq ($(TARGET),)
 ifeq ($(filter $(firstword $(MAKECMDGOALS)),$(IGNORE_OPTS)),)
@@ -1071,6 +1071,9 @@ admin/dyncookie/dyncookie: admin/dyncookie/dyncookie.o
 dev/flags/flags: dev/flags/flags.o
        $(cmd_LD) $(ARCH_FLAGS) $(LDFLAGS) -o $@ $^ $(LDOPTS)
 
+dev/gdb/pm-from-core: dev/gdb/pm-from-core.o
+       $(cmd_LD) $(ARCH_FLAGS) $(LDFLAGS) -o $@ $^ $(LDOPTS)
+
 dev/haring/haring: dev/haring/haring.o
        $(cmd_LD) $(ARCH_FLAGS) $(LDFLAGS) -o $@ $^ $(LDOPTS)
 
@@ -1169,7 +1172,7 @@ distclean: clean
        $(Q)rm -f admin/dyncookie/dyncookie
        $(Q)rm -f dev/haring/haring dev/ncpu/ncpu{,.so} dev/poll/poll dev/tcploop/tcploop
        $(Q)rm -f dev/hpack/decode dev/hpack/gen-enc dev/hpack/gen-rht
-       $(Q)rm -f dev/qpack/decode
+       $(Q)rm -f dev/qpack/decode dev/gdb/pm-from-core
 
 tags:
        $(Q)find src include \( -name '*.c' -o -name '*.h' \) -print0 | \

diff --git a/dev/gdb/pm-from-core.c b/dev/gdb/pm-from-core.c
new file mode 100644 (file)
index 0000000..397d1ef
--- /dev/null
+++ b/dev/gdb/pm-from-core.c
@@ -0,0 +1,141 @@
+/*
+ * 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;
+}