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    SubjectRe: [PATCH] perf tools: Fix build-id matching on vmlinux
    Em Tue, Aug 26, 2014 at 03:38:39PM +0900, Namhyung Kim escreveu:
    > There's a problem on finding correct kernel symbols when perf report
    > runs on a different kernel. Although a part of the problem was solved
    > by the prior commit 0a7e6d1b6844 ("perf tools: Check recorded kernel
    > version when finding vmlinux"), there's a remaining problem still.
    >
    > When perf records samples, it synthesizes the kernel map using
    > machine__mmap_name() and ref_reloc_sym like "[kernel.kallsyms]_text".
    > You can easily see it using 'perf report -D' command.
    >
    > After finishing record, it goes through the recorded events to find
    > maps/dsos actually used. And then record build-id info of them.
    >
    > During this process, it needs to load symbols in a dso and it'd call
    > dso__load_vmlinux() since the default value of the symbol_conf.try_
    > vmlinux_path is true. However it changes dso->long_name to a real
    > path of the vmlinux file (e.g. /lib/modules/3.16.0-rc2+/build/vmlinux)
    > if one is running on a custom kernel.
    >
    > It resulted in that perf report reads the build-id of the vmlinux, but
    > cannot use it since it only knows about the [kernel.kallsyms] map. It
    > then falls back to possible vmlinux paths by using the recorded kernel
    > version (in case of a recent version) or a running kernel silently
    > (which might break the result). I think it's worth going to the
    > stable tree.
    >
    > I can think of a couple of ways to fix it. In this patch, I changed
    > to use the name of "[kernel.kallsyms]" for the kernel build-id event
    > instead of not trying vmlinux paths. This way we can provide maximum
    > info (like annotation) with minimum change IMHO.
    >
    > Before:
    >
    > $ perf record -a usleep 1
    >
    > $ perf buildid-list
    > 00d5ff078efe1d30b8492854f259215fd877ce30 /lib/modules/3.16.0-rc2+/build/vmlinux
    > 78186287bba77069a056a5ccbeb14b7fd2ca3a4b /usr/lib64/libc-2.17.so
    > 4eadca6cb82e0a85edb87c15b5e3980742514501 /usr/lib64/ld-2.17.so
    > 1e272ca30081e81ef41935a630eb2f4c636798b4 /usr/lib64/dri/swrast_dri.so
    >
    > $ perf buildid-list -H
    > 0000000000000000000000000000000000000000 [kernel.kallsyms]
    > 78186287bba77069a056a5ccbeb14b7fd2ca3a4b /usr/lib64/libc-2.17.so
    > 4eadca6cb82e0a85edb87c15b5e3980742514501 /usr/lib64/ld-2.17.so
    > 1e272ca30081e81ef41935a630eb2f4c636798b4 /usr/lib64/dri/swrast_dri.so
    > 0000000000000000000000000000000000000000 /tmp/perf-2523.map
    >
    > After:
    >
    > $ perf record -a usleep 1
    >
    > $ perf buildid-list
    > 00d5ff078efe1d30b8492854f259215fd877ce30 [kernel.kallsyms]

    We are losing information, namely the pathname for the kernel used, that
    may be useful in analysis.

    Why not make sure that if there is a build-id in the perf.data header,
    then we completely refusing anything that doesn't match the build-id?
    I.e. the name is irrelevant for this purpose, the contents, as keyed by
    the build-id, is what matters.

    - Arnaldo

    > 78186287bba77069a056a5ccbeb14b7fd2ca3a4b /usr/lib64/libc-2.17.so
    > 4eadca6cb82e0a85edb87c15b5e3980742514501 /usr/lib64/ld-2.17.so
    > 1e272ca30081e81ef41935a630eb2f4c636798b4 /usr/lib64/dri/swrast_dri.so
    >
    > $ perf buildid-list -H
    > 00d5ff078efe1d30b8492854f259215fd877ce30 [kernel.kallsyms]
    > 78186287bba77069a056a5ccbeb14b7fd2ca3a4b /usr/lib64/libc-2.17.so
    > 4eadca6cb82e0a85edb87c15b5e3980742514501 /usr/lib64/ld-2.17.so
    > 1e272ca30081e81ef41935a630eb2f4c636798b4 /usr/lib64/dri/swrast_dri.so
    > 0000000000000000000000000000000000000000 /tmp/perf-2523.map
    >
    > Cc: stable@vger.kernel.org
    > Signed-off-by: Namhyung Kim <namhyung@kernel.org>
    > ---
    > tools/perf/util/header.c | 3 +++
    > 1 file changed, 3 insertions(+)
    >
    > diff --git a/tools/perf/util/header.c b/tools/perf/util/header.c
    > index 158c787ce0c4..5c4093dee467 100644
    > --- a/tools/perf/util/header.c
    > +++ b/tools/perf/util/header.c
    > @@ -263,6 +263,9 @@ static int __dsos__write_buildid_table(struct list_head *head,
    > machine__mmap_name(machine, nm, sizeof(nm));
    > name = nm;
    > name_len = strlen(nm) + 1;
    > + } else if (dso__is_vmlinux(pos)) {
    > + name = pos->name;
    > + name_len = strlen(pos->name) + 1;
    > } else {
    > name = pos->long_name;
    > name_len = pos->long_name_len + 1;
    > --
    > 2.0.0


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