[PATCH bpf v2 1/3] bpf: don't assume build-id length is always 20 bytes

[Stanislav Fomichev <sdf@google.com>]

Build-id length is not fixed to 20, it can be (`man ld` /--build-id):
  * 128-bit (uuid)
  * 160-bit (sha1)
  * any length specified in ld --build-id=0xhexstring

To fix the issue of missing BPF_STACK_BUILD_ID_VALID for shorter build-ids,
assume that build-id is somewhere in the range of 1 .. 20.
Set the remaining bytes to zero.

v2:
* don't introduce new "len = min(BPF_BUILD_ID_SIZE, nhdr->n_descsz)",
  we already know that nhdr->n_descsz <= BPF_BUILD_ID_SIZE if we enter
  this 'if' condition

Fixes: 615755a77b24 ("bpf: extend stackmap to save binary_build_id+offset instead of address")
Signed-off-by: Stanislav Fomichev <sdf@google.com>
---
 kernel/bpf/stackmap.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/kernel/bpf/stackmap.c b/kernel/bpf/stackmap.c
index d9e2483669d0..f9df545e92f6 100644
--- a/kernel/bpf/stackmap.c
+++ b/kernel/bpf/stackmap.c
@@ -180,11 +180,14 @@ static inline int stack_map_parse_build_id(void *page_addr,
 
 		if (nhdr->n_type == BPF_BUILD_ID &&
 		    nhdr->n_namesz == sizeof("GNU") &&
-		    nhdr->n_descsz == BPF_BUILD_ID_SIZE) {
+		    nhdr->n_descsz > 0 &&
+		    nhdr->n_descsz <= BPF_BUILD_ID_SIZE) {
 			memcpy(build_id,
 			       note_start + note_offs +
 			       ALIGN(sizeof("GNU"), 4) + sizeof(Elf32_Nhdr),
-			       BPF_BUILD_ID_SIZE);
+			       nhdr->n_descsz);
+			memset(build_id + nhdr->n_descsz, 0,
+			       BPF_BUILD_ID_SIZE - nhdr->n_descsz);
 			return 0;
 		}
 		new_offs = note_offs + sizeof(Elf32_Nhdr) +
-- 
2.20.1.97.g81188d93c3-goog