[PATCH bpf-next] libbpf: don't adjust USDT semaphore address if .stapsdt.base addr is missing

[PATCH bpf-next] libbpf: don't adjust USDT semaphore address if .stapsdt.base addr is missing

[Andrii Nakryiko]

USDT ELF note optionally can record an offset of .stapsdt.base, which is
used to make adjustments to USDT target attach address. Currently,
libbpf will do this address adjustment unconditionally if it finds
.stapsdt.base ELF section in target binary. But there is a corner case
where .stapsdt.base ELF section is present, but specific USDT note
doesn't reference it. In such case, libbpf will basically just add base
address and end up with absolutely incorrect USDT target address.

This adjustment has to be done only if both .stapsdt.sema section is
present and USDT note is recording a reference to it.

Fixes: 74cc6311cec9 ("libbpf: Add USDT notes parsing and resolution logic")
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
---
 tools/lib/bpf/usdt.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tools/lib/bpf/usdt.c b/tools/lib/bpf/usdt.c
index 5f085736c6c4..4e4a52742b01 100644
--- a/tools/lib/bpf/usdt.c
+++ b/tools/lib/bpf/usdt.c
@@ -661,7 +661,7 @@ static int collect_usdt_targets(struct usdt_manager *man, Elf *elf, const char *
 		 *   [0] https://sourceware.org/systemtap/wiki/UserSpaceProbeImplementation
 		 */
 		usdt_abs_ip = note.loc_addr;
-		if (base_addr)
+		if (base_addr && note.base_addr)
 			usdt_abs_ip += base_addr - note.base_addr;
 
 		/* When attaching uprobes (which is what USDTs basically are)
-- 
2.43.5

Re: [PATCH bpf-next] libbpf: don't adjust USDT semaphore address if .stapsdt.base addr is missing

[Jiri Olsa]

On Thu, Nov 21, 2024 at 02:45:58PM -0800, Andrii Nakryiko wrote:
> USDT ELF note optionally can record an offset of .stapsdt.base, which is
> used to make adjustments to USDT target attach address. Currently,
> libbpf will do this address adjustment unconditionally if it finds
> .stapsdt.base ELF section in target binary. But there is a corner case
> where .stapsdt.base ELF section is present, but specific USDT note
> doesn't reference it. In such case, libbpf will basically just add base
> address and end up with absolutely incorrect USDT target address.
> 
> This adjustment has to be done only if both .stapsdt.sema section is
> present and USDT note is recording a reference to it.
> 
> Fixes: 74cc6311cec9 ("libbpf: Add USDT notes parsing and resolution logic")
> Signed-off-by: Andrii Nakryiko <andrii@kernel.org>

nice, lgtm

Acked-by: Jiri Olsa <jolsa@kernel.org>

jirka

> ---
>  tools/lib/bpf/usdt.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/tools/lib/bpf/usdt.c b/tools/lib/bpf/usdt.c
> index 5f085736c6c4..4e4a52742b01 100644
> --- a/tools/lib/bpf/usdt.c
> +++ b/tools/lib/bpf/usdt.c
> @@ -661,7 +661,7 @@ static int collect_usdt_targets(struct usdt_manager *man, Elf *elf, const char *
>  		 *   [0] https://sourceware.org/systemtap/wiki/UserSpaceProbeImplementation
>  		 */
>  		usdt_abs_ip = note.loc_addr;
> -		if (base_addr)
> +		if (base_addr && note.base_addr)
>  			usdt_abs_ip += base_addr - note.base_addr;
>  
>  		/* When attaching uprobes (which is what USDTs basically are)
> -- 
> 2.43.5
> 
> 

Re: [PATCH bpf-next] libbpf: don't adjust USDT semaphore address if .stapsdt.base addr is missing

[patchwork-bot+netdevbpf]

Hello:

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

On Thu, 21 Nov 2024 14:45:58 -0800 you wrote:
> USDT ELF note optionally can record an offset of .stapsdt.base, which is
> used to make adjustments to USDT target attach address. Currently,
> libbpf will do this address adjustment unconditionally if it finds
> .stapsdt.base ELF section in target binary. But there is a corner case
> where .stapsdt.base ELF section is present, but specific USDT note
> doesn't reference it. In such case, libbpf will basically just add base
> address and end up with absolutely incorrect USDT target address.
> 
> [...]

Here is the summary with links:
  - [bpf-next] libbpf: don't adjust USDT semaphore address if .stapsdt.base addr is missing
    https://git.kernel.org/bpf/bpf-next/c/d00058e676a9

You are awesome, thank you!
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