[PATCH v2 bpf-next 02/10] lib/buildid: take into account e_phoff when fetching program headers

[Andrii Nakryiko <andrii@xxxxxxxxxx>]

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@xxxxxxxxx>
Signed-off-by: Andrii Nakryiko <andrii@xxxxxxxxxx>
---
 lib/buildid.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/lib/buildid.c b/lib/buildid.c
index 1442a2483a8b..ce48ffab4111 100644
--- a/lib/buildid.c
+++ b/lib/buildid.c
@@ -206,7 +206,7 @@ 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;
+	__u32 phnum, phoff, i;
 
 	ehdr = freader_fetch(r, 0, sizeof(Elf32_Ehdr));
 	if (!ehdr)
@@ -214,13 +214,14 @@ static int get_build_id_32(struct freader *r, unsigned char *build_id, __u32 *si
 
 	/* subsequent freader_fetch() calls invalidate pointers, so remember locally */
 	phnum = 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;
 
@@ -237,6 +238,7 @@ 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)
@@ -244,13 +246,14 @@ static int get_build_id_64(struct freader *r, unsigned char *build_id, __u32 *si
 
 	/* subsequent freader_fetch() calls invalidate pointers, so remember locally */
 	phnum = 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