[PATCH v3 bpf-next 03/10] lib/buildid: take into account e_phoff when fetching program headers

[Andrii Nakryiko <andrii@kernel.org>]

Current code assumption is that program (segment) headers are following
ELF header immediately. This is a common case, but is not guaranteed. So
take into account e_phoff field of the ELF header when accessing program
headers.

Reported-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
---
 lib/buildid.c | 26 ++++++++------------------
 1 file changed, 8 insertions(+), 18 deletions(-)

diff --git a/lib/buildid.c b/lib/buildid.c
index 522850f827a5..ba453a3784d1 100644
--- a/lib/buildid.c
+++ b/lib/buildid.c
@@ -207,27 +207,22 @@ static int get_build_id_32(struct freader *r, unsigned char *build_id, __u32 *si
 	const Elf32_Ehdr *ehdr;
 	const Elf32_Phdr *phdr;
 	__u32 phnum, i;
+	__u64 phoff;
 
 	ehdr = freader_fetch(r, 0, sizeof(Elf32_Ehdr));
 	if (!ehdr)
 		return r->err;
 
-	/*
-	 * FIXME
-	 * Neither ELF spec nor ELF loader require that program headers
-	 * start immediately after ELF header.
-	 */
-	if (ehdr->e_phoff != sizeof(Elf32_Ehdr))
-		return -EINVAL;
-
 	/* subsequent freader_fetch() calls invalidate pointers, so remember locally */
 	phnum = READ_ONCE(ehdr->e_phnum);
+	phoff = READ_ONCE(ehdr->e_phoff);
+
 	/* only supports phdr that fits in one page */
 	if (phnum > (PAGE_SIZE - sizeof(Elf32_Ehdr)) / sizeof(Elf32_Phdr))
 		return -EINVAL;
 
 	for (i = 0; i < phnum; ++i) {
-		phdr = freader_fetch(r, i * sizeof(Elf32_Phdr), sizeof(Elf32_Phdr));
+		phdr = freader_fetch(r, phoff + i * sizeof(Elf32_Phdr), sizeof(Elf32_Phdr));
 		if (!phdr)
 			return r->err;
 
@@ -245,27 +240,22 @@ static int get_build_id_64(struct freader *r, unsigned char *build_id, __u32 *si
 	const Elf64_Ehdr *ehdr;
 	const Elf64_Phdr *phdr;
 	__u32 phnum, i;
+	__u64 phoff;
 
 	ehdr = freader_fetch(r, 0, sizeof(Elf64_Ehdr));
 	if (!ehdr)
 		return r->err;
 
-	/*
-	 * FIXME
-	 * Neither ELF spec nor ELF loader require that program headers
-	 * start immediately after ELF header.
-	 */
-	if (ehdr->e_phoff != sizeof(Elf64_Ehdr))
-		return -EINVAL;
-
 	/* subsequent freader_fetch() calls invalidate pointers, so remember locally */
 	phnum = READ_ONCE(ehdr->e_phnum);
+	phoff = READ_ONCE(ehdr->e_phoff);
+
 	/* only supports phdr that fits in one page */
 	if (phnum > (PAGE_SIZE - sizeof(Elf64_Ehdr)) / sizeof(Elf64_Phdr))
 		return -EINVAL;
 
 	for (i = 0; i < phnum; ++i) {
-		phdr = freader_fetch(r, i * sizeof(Elf64_Phdr), sizeof(Elf64_Phdr));
+		phdr = freader_fetch(r, phoff + i * sizeof(Elf64_Phdr), sizeof(Elf64_Phdr));
 		if (!phdr)
 			return r->err;
 
-- 
2.43.0