linux-kernel - Re: [PATCH RESEND 2/3] perf/core: Fix MMAP event path names with backing files

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Message-ID: <CAOQ4uxjip6vRZq+ZWYemsOaeXvQ7LRmJWUnzF7R+53wYdeLNGA@mail.gmail.com>
Date: Mon, 13 Oct 2025 12:43:19 +0200
From: Amir Goldstein <amir73il@...il.com>
To: Adrian Hunter <adrian.hunter@...el.com>
Cc: Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>, 
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>, 
	Arnaldo Carvalho de Melo <acme@...nel.org>, Jiri Olsa <jolsa@...nel.org>, Namhyung Kim <namhyung@...nel.org>, 
	Mark Rutland <mark.rutland@....com>, Ian Rogers <irogers@...gle.com>, 
	Mathieu Poirier <mathieu.poirier@...aro.org>, Miklos Szeredi <miklos@...redi.hu>, 
	Alexander Viro <viro@...iv.linux.org.uk>, Christian Brauner <brauner@...nel.org>, 
	Edd Barrett <edd@...unixzoo.co.uk>, Laurence Tratt <laurie@...tt.net>, linux-kernel@...r.kernel.org, 
	linux-perf-users@...r.kernel.org
Subject: Re: [PATCH RESEND 2/3] perf/core: Fix MMAP event path names with
 backing files

On Mon, Oct 13, 2025 at 9:23 AM Adrian Hunter <adrian.hunter@...el.com> wrote:
>
> Some file systems like FUSE-based ones or overlayfs may record the backing
> file in struct vm_area_struct vm_file, instead of the user file that the
> user mmapped.
>
> Since commit def3ae83da02f ("fs: store real path instead of fake path in
> backing file f_path"), file_path() no longer returns the user file path
> when applied to a backing file.  There is an existing helper
> file_user_path() for that situation.
>
> Use file_user_path() instead of file_path() to get the path for MMAP
> and MMAP2 events.
>
> Example:
>
>   Setup:
>
>     # cd /root
>     # mkdir test ; cd test ; mkdir lower upper work merged
>     # cp `which cat` lower
>     # mount -t overlay overlay -olowerdir=lower,upperdir=upper,workdir=work merged
>     # perf record -e intel_pt//u -- /root/test/merged/cat /proc/self/maps
>     ...
>     55b0ba399000-55b0ba434000 r-xp 00018000 00:1a 3419                       /root/test/merged/cat
>     ...
>     [ perf record: Woken up 1 times to write data ]
>     [ perf record: Captured and wrote 0.060 MB perf.data ]
>     #
>
>   Before:
>
>     File name is wrong (/cat), so decoding fails:
>
>     # perf script --no-itrace --show-mmap-events
>              cat     367 [016]   100.491492: PERF_RECORD_MMAP2 367/367: [0x55b0ba399000(0x9b000) @ 0x18000 00:02 3419 489959280]: r-xp /cat
>     ...
>     # perf script --itrace=e | wc -l
>     Warning:
>     19 instruction trace errors
>     19
>     #
>
>   After:
>
>     File name is correct (/root/test/merged/cat), so decoding is ok:
>
>     # perf script --no-itrace --show-mmap-events
>                  cat     364 [016]    72.153006: PERF_RECORD_MMAP2 364/364: [0x55ce4003d000(0x9b000) @ 0x18000 00:02 3419 3132534314]: r-xp /root/test/merged/cat
>     # perf script --itrace=e
>     # perf script --itrace=e | wc -l
>     0
>     #
>
> Fixes: def3ae83da02f ("fs: store real path instead of fake path in backing file f_path")
> Cc: stable@...r.kernel.org
> Signed-off-by: Adrian Hunter <adrian.hunter@...el.com>

Reviewed-by: Amir Goldstein <amir73il@...il.com>

> ---
>  kernel/events/core.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index cd63ec84e386..7b5c2373a8d7 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -9416,7 +9416,7 @@ static void perf_event_mmap_event(struct perf_mmap_event *mmap_event)
>                  * need to add enough zero bytes after the string to handle
>                  * the 64bit alignment we do later.
>                  */
> -               name = file_path(file, buf, PATH_MAX - sizeof(u64));
> +               name = d_path(file_user_path(file), buf, PATH_MAX - sizeof(u64));
>                 if (IS_ERR(name)) {
>                         name = "//toolong";
>                         goto cpy_name;
> --
> 2.48.1
>