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Re: [PATCH] Remove unnecessary IFUNC dispatch for __memset_chk.
- From: "H.J. Lu" <hjl dot tools at gmail dot com>
- To: Zack Weinberg <zackw at panix dot com>
- Cc: Andreas Schwab <schwab at linux-m68k dot org>, GNU C Library <libc-alpha at sourceware dot org>
- Date: Sun, 9 Aug 2015 08:39:00 -0700
- Subject: Re: [PATCH] Remove unnecessary IFUNC dispatch for __memset_chk.
- Authentication-results: sourceware.org; auth=none
- References: <20150809013434 dot 0B16814B9A at panix1 dot panix dot com> <m28u9lotfk dot fsf at linux-m68k dot org> <55C76FCD dot 5020607 at panix dot com>
On Sun, Aug 9, 2015 at 8:20 AM, Zack Weinberg <firstname.lastname@example.org> wrote:
> On 08/09/2015 02:41 AM, Andreas Schwab wrote:
>> Zack Weinberg <email@example.com> writes:
>>> __memset_chk does a bounds check (which the C compiler will optimize
>>> just fine) and then calls memset. That call will itself go through
>>> IFUNC dispatch,
>> Does it?
> On further investigation it appears not to -- specifically, internal
> calls using __GI_foo appear to go straight to the default implementation
> of 'foo'.
> If so, I am inclined to think that that is a bug -- there are a *lot* of
> internal calls to memset and memcpy in libc, they should not miss out on
> architectural tuning. I don't particularly understand how IFUNC works,
> but wouldn't it be sufficient to send internal calls to anything with an
> IFUNC through the PLT? (I suppose there would then be a question of
> whether the architectural optimizations made up for the PLT overhead.)
Here is a description of IFUNC:
> I am also inclined to think that there is no good reason to gunk up the
> fortify implementation with IFUNCed _chk functions, regardless. If
> losing out on architecturally tuned memory block functions is acceptable
> for normal calls inside libc, it should also be acceptable for fortified
> applications (which one *expects* to incur a cost).
The purpose of multiarch/memset_chk.S is to avoid one extra indirect
call via PLT.