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linux-user: Fix AT_PHDR when program headers are relocated into their own segment

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When a binary is patched or relocated such that the program header table is
moved into a separate PT_LOAD segment (rather than sitting at the start of the
first loadable segment), QEMU's AT_PHDR auxv entry becomes incorrect. The
loader was computing AT_PHDR as load_addr + e_phoff, which assumes the headers
are mapped 1:1 from file offset 0. This breaks when the headers are elsewhere.

The Linux kernel instead locates the PT_LOAD segment that contains e_phoff,
then computes the in-memory address as p_vaddr + (e_phoff - p_offset). This
correctly handles relocated headers.

Fix by:
1. Add phdr_addr field to image_info to cache the resolved address.
2. Initialize to load_addr + e_phoff (fallback for headers outside any PT_LOAD).
3. In the PT_LOAD mapping loop, detect if the segment contains e_phoff and
   override with the segment-relative address.
4. Use info->phdr_addr for AT_PHDR instead of the incorrect formula.

Signed-off-by: Akshit Yadav <valium7171@gmail.com>
Reviewed-by: Helge Deller <deller@gmx.de>
This commit is contained in:
Akshit Yadav
2026-06-13 20:51:56 +05:30
committed by Helge Deller
parent c7cf7c8101
commit 156e536a7b
2 changed files with 21 additions and 1 deletions
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21
linux-user/elfload.c
View File

@@ -699,7 +699,7 @@ static abi_ulong create_elf_tables(abi_ulong p, int argc, int envc,
/* There must be exactly DLINFO_ITEMS entries here, or the assert
* on info->auxv_len will trigger.
*/
NEW_AUX_ENT(AT_PHDR, (abi_ulong)(info->load_addr + exec->e_phoff));
NEW_AUX_ENT(AT_PHDR, (abi_ulong)(info->phdr_addr));
NEW_AUX_ENT(AT_PHENT, (abi_ulong)(sizeof (struct elf_phdr)));
NEW_AUX_ENT(AT_PHNUM, (abi_ulong)(exec->e_phnum));
NEW_AUX_ENT(AT_PAGESZ, (abi_ulong)(TARGET_PAGE_SIZE));
@@ -1469,6 +1469,12 @@ static void load_elf_image(const char *image_name, const ImageSource *src,
info->data_offset = load_bias;
info->load_addr = load_addr;
info->entry = ehdr->e_entry + load_bias;
/*
* Fallback for AT_PHDR if the program headers do not fall within
* any PT_LOAD segment (see the loop below, which overrides this with
* the correct in-memory address when a containing segment is found).
*/
info->phdr_addr = load_addr + ehdr->e_phoff;
info->start_code = -1;
info->end_code = 0;
info->start_data = -1;
@@ -1523,6 +1529,19 @@ static void load_elf_image(const char *image_name, const ImageSource *src,
vaddr_ef = vaddr + eppnt->p_filesz;
vaddr_em = vaddr + eppnt->p_memsz;
/*
* If this segment contains the program headers, record their
* in-memory address for AT_PHDR. This matches the kernel, which
* locates the headers via the containing PT_LOAD rather than
* assuming load_addr + e_phoff (false when the phdrs are not
* mapped 1:1 from file offset 0, e.g. relocated into their own
* segment by a binary patcher).
*/
if (eppnt->p_offset <= ehdr->e_phoff &&
ehdr->e_phoff < eppnt->p_offset + eppnt->p_filesz) {
info->phdr_addr = vaddr + (ehdr->e_phoff - eppnt->p_offset);
}
/*
* Some segments may be completely empty, with a non-zero p_memsz
* but no backing file segment.

1
linux-user/qemu.h
View File

@@ -26,6 +26,7 @@
struct image_info {
abi_ulong load_bias;
abi_ulong load_addr;
abi_ulong phdr_addr;
abi_ulong start_code;
abi_ulong end_code;
abi_ulong start_data;