Re: [patch] Correct range checking in mallopt/mxfast/tcache [BZ #25194]

[Jonathan Nieder <jrnieder at gmail dot com>]

DJ Delorie wrote:
> Jonathan Nieder <jrnieder@gmail.com> writes:

>> nit: unsigned overflow is defined in C to wrap around, so we don't
>> have to worry about undefined behavior here.
>
> Including implied overflow by type cast conversions?

Yes, C99 section 6.3.1.3 describes signed-to-unsigned conversions.
Thanks for asking, though, since that wasn't the case I was thinking
of before.

Walking through the caller, we're talking about TUNABLE_SET, which does

  __tunable_set_val (TUNABLE_ENUM_NAME (__top, __ns, __id),		      \
			& (__type) {__val});				      \

The only caller for TUNABLE_SET I see is do_tunable_update_val, which
bounds-checks its input.

>> This also changes the return value from these helpers.  Forgive my
>> ignorance: where do they get called?  I assumed it would be
>> __libc_mallopt, but I don't see any calls from there.
>
> The only two places they could be called from is mallopt() and the
> tunables macros in arena.c.  The tunables code ignores any return value.

Ah --- thanks for the pointer.  (Not about this patch: a comment
mentioning the arena.c tunables macros would be handy for the unwary
reader.)

So the only intended callers are set_tcache_max and set_mxfast (today)
and __libc_mallopt (in the future)?  Makes sense.

Thanks,
Jonathan