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    SubjectRe: [PATCH] trace-cmd: prevent print_graph_duration buffer overflow
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    On Sun, 13 Jun 2010 17:01:34 EDT, Chase Douglas said:
    > On Sun, 2010-06-13 at 16:52 -0400, Valdis.Kletnieks@vt.edu wrote:
    > > On Sun, 13 Jun 2010 13:11:48 EDT, Chase Douglas said:
    > > > Passing n > sizeof(string) to snprintf can cause a glibc buffer overflow
    > > > condition. We know the exact size of nsecs_str, so use it instead of
    > > > math that may overflow.
    > >
    > > > /* Print nsecs (we don't want to exceed 7 numbers) */
    > > > if ((s->len - len) < 7) {
    > > > - snprintf(nsecs_str, 8 - (s->len - len), "%03lu", nsecs_rem);
    > > > + snprintf(nsecs_str, sizeof(nsecs_str), "%03lu", nsecs_rem);
    > >
    > > We only get into this code after we've checked that the length is under 7
    > > characters. How much overflow can happen as long as the sizeof(nsecs_str) is a
    > > sane size (like at least 8 chars)? Probably a better bet would be doing the
    > > right thing and 'BUILD_BUG_ON(sizeof(nsecs_str) < 8);'?
    >
    > nsecs_str is a local variable defined just above this block of code as:
    >
    > char nsecs_str[5];
    >
    > I was hitting cases where s->len == 64 and len == 63, leading to the
    > size argument of snprintf being 7 on a 5 byte string. I didn't delve too
    > much into the reasoning for the if statement, but I think it's math is
    > not actually related to the size of nsecs_rem but to some other string
    > length.

    This is starting to smell like that patch is just papering over a bug...

    I saw that '8 -' and made the rash assumption that was the size of the array.
    Is 5 in fact big enough and the 's->len - len' calculation is broken, or
    should it be bigger? As you noted, that length calculation is looking a tad
    sketchy. (And if we're stuck with '5' because it's a magic number for
    somebody's formatting purposes, maybe it needs to be a #define?)

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