Re: [PATCH 3/5] pahole: Honour exclusive BTF loading

[Alan Maguire <alan.maguire@oracle.com>]

On 19/11/2024 13:40, Arnaldo Carvalho de Melo wrote:
> From: Arnaldo Carvalho de Melo <acme@redhat.com>
> 
> When we specify 'btf' and BTF info is available in the system, in the
> /sys/kernel/btf/vmlinux file, pahole will find it and use it to get the
> type information asked for:
> 
>   root@x1:~# perf trace -o /tmp/output -e openat pahole -F btf list_head ; tail -1 /tmp/output
>   struct list_head {
>   	struct list_head *         next;                 /*     0     8 */
>   	struct list_head *         prev;                 /*     8     8 */
> 
>   	/* size: 16, cachelines: 1, members: 2 */
>   	/* last cacheline: 16 bytes */
>   };
> 
>        1.579 ( 0.010 ms): pahole/764777 openat(dfd: CWD, filename: "/sys/kernel/btf/vmlinux", flags: RDONLY|CLOEXEC) = 3
>   root@x1:~#
> 
> But, if the system doesn't have BTF info, which we now can simulate by using an
> environment variable it is tryng DWARF after failing for BTF, which isn't what
> the user asked for having specified just btf in the -F pahole command line:
> 
>   root@x1:~# export PAHOLE_VMLINUX_BTF_FILENAME=non-existent
>   root@x1:~# perf trace -o /tmp/output -e openat,access pahole -F btf list_head ; tail -12 /tmp/output
>   struct list_head {
>   	struct list_head *         next;                 /*     0     8 */
>   	struct list_head *         prev;                 /*     8     8 */
> 
>   	/* size: 16, cachelines: 1, members: 2 */
>   	/* last cacheline: 16 bytes */
>   };
> 
>   1.643 ( 0.003 ms): pahole/765220 access(filename: "list_head", mode: R)      = -1 ENOENT (No such file or directory)
>   1.658 ( 0.002 ms): pahole/765220 access(filename: "non-existent", mode: R)   = -1 ENOENT (No such file or directory)
>   1.697 ( 0.018 ms): pahole/765220 openat(dfd: CWD, filename: "/sys/kernel/notes") = 3
>   1.801 ( 0.004 ms): pahole/765220 openat(dfd: CWD, filename: "vmlinux")       = -1 ENOENT (No such file or directory)
>   1.807 ( 0.005 ms): pahole/765220 openat(dfd: CWD, filename: "/boot/vmlinux") = -1 ENOENT (No such file or directory)
>   1.927 ( 0.005 ms): pahole/765220 openat(dfd: CWD, filename: "/usr/lib/debug/lib/modules/6.11.6-200.fc40.x86_64/vmlinux", flags: RDONLY|CLOEXEC) = 3
>   root@x1:~#
> 
> So honour the request and fails when just BTF is asked and no BTF file is
> found:
> 
>   # export PAHOLE_VMLINUX_BTF_FILENAME=non-existent
>   # pahole -F btf list_head
>   pahole: couldn't find any btf debug information on this system.
>   #
> 
> Reported-by: Matthias Schwarzott <zzam@gentoo.org>
> Cc: Alan Maguire <alan.maguire@oracle.com>
> Cc: Andrii Nakryiko <andrii@kernel.org>
> Cc: Eduard Zingerman <eddyz87@gmail.com>
> Cc: Jiri Olsa <jolsa@kernel.org>
> Cc: Song Liu <song@kernel.org>
> Cc: Yonghong Song <yonghong.song@linux.dev>
> Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

For the series (since some of my replies didn't reach the list, apologies)

Reviewed-by: Alan Maguire <alan.maguire@oracle.com>
Tested-by: Alan Maguire <alan.maguire@oracle.com>