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

[Arnaldo Carvalho de Melo <acme@kernel.org>]

On Tue, Nov 19, 2024 at 05:47:21PM +0000, Alan Maguire wrote:
> 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>