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    Subject[PATCH] perf: Incorrect use of snprintf results in SEGV

    I have a workload where perf top scribbles over the stack and we
    SEGV. What makes it interesting is that an snprintf is causing this.

    The workload is a c++ gem that has method names over 3000 characters
    long, but snprintf is designed to avoid overrunning buffers. So what
    went wrong?

    The problem is we assume snprintf returns the number of characters

    ret += repsep_snprintf(bf + ret, size - ret, "[%c] ", self->level);
    ret += repsep_snprintf(bf + ret, size - ret, "%s", self->ms.sym->name);

    Unfortunately this is not how snprintf works. snprintf returns the
    number of characters that would have been written if there was enough
    space. In the above case, if the first snprintf returns a value larger
    than size, we pass a negative size into the second snprintf and
    happily scribble over the stack. If you have 3000 character c++
    methods thats a lot of stack to trample.

    This patch fixes repsep_snprintf by clamping the value at size - 1
    which is the maximum snprintf can write before adding the NULL

    I get the sinking feeling that there are a lot of other uses of
    snprintf that have this same bug, we should audit them all.

    Signed-off-by: Anton Blanchard <>

    Index: linux-build/tools/perf/util/sort.c
    --- linux-build.orig/tools/perf/util/sort.c 2012-03-07 10:58:57.502318907 +1100
    +++ linux-build/tools/perf/util/sort.c 2012-03-07 11:00:58.812423271 +1100
    @@ -33,6 +33,9 @@ static int repsep_snprintf(char *bf, siz
    + if (n >= (int)size)
    + return size - 1;
    return n;

     \ /
      Last update: 2012-03-07 01:45    [W:0.019 / U:10.512 seconds]
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