[PATCH bpf-next] libbpf: fix sym_is_subprog() logic for weak global subprogs

[PATCH bpf-next] libbpf: fix sym_is_subprog() logic for weak global subprogs

[Andrii Nakryiko]

sym_is_subprog() is incorrectly rejecting relocations against *weak*
global subprogs. Fix that by realizing that STB_WEAK is also a global
function.

While it seems like verifier doesn't support taking an address of
non-static subprog right now, it's still best to fix support for it on
libbpf side, otherwise users will get a very confusing error during BPF
skeleton generation or static linking due to misinterpreted relocation:

  libbpf: prog 'handle_tp': bad map relo against 'foo' in section '.text'
  Error: failed to open BPF object file: Relocation failed

It's clearly not a map relocation, but is treated and reported as such
without this fix.

Fixes: 53eddb5e04ac ("libbpf: Support subprog address relocation")
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
---
 tools/lib/bpf/libbpf.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index 712b95e8891b..05ad264ff09b 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -4013,7 +4013,7 @@ static bool sym_is_subprog(const Elf64_Sym *sym, int text_shndx)
 		return true;
 
 	/* global function */
-	return bind == STB_GLOBAL && type == STT_FUNC;
+	return (bind == STB_GLOBAL || bind == STB_WEAK) && type == STT_FUNC;
 }
 
 static int find_extern_btf_id(const struct btf *btf, const char *ext_name)
-- 
2.43.5

Re: [PATCH bpf-next] libbpf: fix sym_is_subprog() logic for weak global subprogs

[patchwork-bot+netdevbpf]

Hello:

This patch was applied to bpf/bpf-next.git (master)
by Alexei Starovoitov <ast@kernel.org>:

On Tue,  8 Oct 2024 18:15:54 -0700 you wrote:
> sym_is_subprog() is incorrectly rejecting relocations against *weak*
> global subprogs. Fix that by realizing that STB_WEAK is also a global
> function.
> 
> While it seems like verifier doesn't support taking an address of
> non-static subprog right now, it's still best to fix support for it on
> libbpf side, otherwise users will get a very confusing error during BPF
> skeleton generation or static linking due to misinterpreted relocation:
> 
> [...]

Here is the summary with links:
  - [bpf-next] libbpf: fix sym_is_subprog() logic for weak global subprogs
    https://git.kernel.org/bpf/bpf-next/c/4073213488be

You are awesome, thank you!
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