Re: [PATCH 1/5] perf dso: Guard against errno==0 when dso__get_filename() returns NULL

[David Laight <david.laight.linux@gmail.com>]

On Sun,  2 Aug 2026 11:20:18 -0300
Arnaldo Carvalho de Melo <acme@kernel.org> wrote:

> From: Arnaldo Carvalho de Melo <acme@redhat.com>
> 
> __open_dso() computes fd = -errno when dso__get_filename() returns NULL.
> Some failure paths in dso__get_filename() (e.g. binary type mismatch)
> return NULL without making a syscall, leaving errno at 0 from a prior
> successful call.

Except that errno isn't set to zero by successful syscalls.
It is only ever set by ones that fail.
The application may set errno to zero (eg to detect errors from some
library functions).

So paths that don't make syscalls will be returning a 'random' errno.

	David

>  fd = -0 = 0, which is stdin — subsequent code treats
> it as a valid file descriptor.
> 
> Fall back to ENOENT when errno is 0, ensuring fd is always negative on
> failure.
> 
> Fixes: eba5102d2f0b ("perf tools: Add global list of opened dso objects")
> Reported-by: sashiko-bot <sashiko-bot@kernel.org>
> Cc: Jiri Olsa <jolsa@kernel.org>
> Assisted-by: Claude:claude-opus-4.6
> Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
> ---
>  tools/perf/util/dso.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/tools/perf/util/dso.c b/tools/perf/util/dso.c
> index 2309196d8df3111c..e087a89066bdbc02 100644
> --- a/tools/perf/util/dso.c
> +++ b/tools/perf/util/dso.c
> @@ -643,7 +643,7 @@ static int __open_dso(struct dso *dso, struct machine *machine)
>  	if (name)
>  		fd = do_open(name);
>  	else
> -		fd = -errno;
> +		fd = errno ? -errno : -ENOENT;
>  
>  	if (decomp)
>  		unlink(name);