linux-kernel - Re: [GIT PULL] tracing: Updates for 5.2

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Message-ID: <CAHk-=wihYB8w__YQjgYjYZsVniu5CtkTcFycmCGdqVg8GUje7g@mail.gmail.com>
Date:   Wed, 15 May 2019 16:29:50 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [GIT PULL] tracing: Updates for 5.2

On Wed, May 15, 2019 at 10:36 AM Steven Rostedt <rostedt@...dmis.org> wrote:
>
> The major changes in this tracing update includes:

This is not directly related to this pull request, but newer versions
of gcc hate your trace_iterator clearing trick.

This code:

                /* reset all but tr, trace, and overruns */
                memset(&iter.seq, 0,
                       sizeof(struct trace_iterator) -
                       offsetof(struct trace_iterator, seq));

not only has a completely misleading comment (it resets a lot more
than the comment states), but modern gcc looks at that code and says
"oh, you're passing it a pointer to 'iter.seq', but then clearing a
lot more than a 'trace_seq'":

  In function ‘memset’,
      inlined from ‘ftrace_dump’ at kernel/trace/trace.c:8914:3:
 /include/linux/string.h:344:9: warning: ‘__builtin_memset’ offset
[8505, 8560] from the object at ‘iter’ is out of the bounds of
referenced subobject ‘seq’ with type ‘struct trace_seq’ at offset 4368
[-Warray-bounds]
    344 |  return __builtin_memset(p, c, size);
        |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~

It's a somewhat annoying warning because the code itself is
technically correct, but at the same time, I think the gcc warning is
reasonable. You *are* passing it a 'struct trace_seq' pointer, and
then you're clearing a whole lot more than that.

One option is to just rewrite it something like

        const unsigned int offset = offsetof(struct trace_iterator, seq);
        memset(offset+(void *)&iter, 0, sizeof(iter) - offset);

which should compile cleanly - because now you're doing the memset on
a part of the much bigger 'iter' structure, not on one member (and
overflowing that one member).

Another option might be to separate the zeroed part of the structure
into a sub-structure of its own, and then just use

        memset(&iter.sub, 0, sizeof(iter.sub));

but then you'd obviously have to change all the uses of the sub-fields..

                      Linus