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Re: [PATCH] Remove unnecessary IFUNC dispatch for __memset_chk.
- From: Alexander Monakov <amonakov at ispras dot ru>
- To: "H.J. Lu" <hjl dot tools at gmail dot com>
- Cc: Ondřej Bílka <neleai at seznam dot cz>, Zack Weinberg <zackw at panix dot com>, Andreas Schwab <schwab at linux-m68k dot org>, GNU C Library <libc-alpha at sourceware dot org>
- Date: Mon, 17 Aug 2015 23:04:00 +0300 (MSK)
- Subject: Re: [PATCH] Remove unnecessary IFUNC dispatch for __memset_chk.
- Authentication-results: sourceware.org; auth=none
- References: <55C79AD8 dot 3070301 at panix dot com> <20150811203001 dot GD4134 at domone> <CAMe9rOos4HTn3VhgK44JnFo+95Ph5xzWQTheP1EpoCM7=8XeoQ at mail dot gmail dot com> <alpine dot LNX dot 2 dot 20 dot 1508171837140 dot 10765 at monopod dot intra dot ispras dot ru> <CAMe9rOqOvjFvstHnCzKs7mShAH=EiftzzdpstiW_i9ou7QMgCA at mail dot gmail dot com> <alpine dot LNX dot 2 dot 20 dot 1508171932010 dot 10765 at monopod dot intra dot ispras dot ru> <CAMe9rOprUnCei8i9X+eLoWCXA_sb5VY9m_x-3uYHpt4Cy+8YjQ at mail dot gmail dot com> <alpine dot LNX dot 2 dot 20 dot 1508171956100 dot 10765 at monopod dot intra dot ispras dot ru> <CAMe9rOr=s41ia=o1Q9bXpw0UvQ5DjW=Wk8r+teuC+05LXxoEpg at mail dot gmail dot com> <alpine dot LNX dot 2 dot 20 dot 1508172051410 dot 21925 at monopod dot intra dot ispras dot ru> <CAMe9rOqKa+U-FSw=E2oyTo_KCPZ_DS=0rFhD-bLFPGT2MVarow at mail dot gmail dot com> <alpine dot LNX dot 2 dot 20 dot 1508172102220 dot 21925 at monopod dot intra dot ispras dot ru> <CAMe9rOpuGM21oBg=00fkK=JXPP3tOwoN_NG1kxuwrX5Gx02WUA at mail dot gmail dot com> <alpine dot LNX dot 2 dot 20 dot 1508172233370 dot 21925 at monopod dot intra dot ispras dot ru> <CAMe9rOr4OaEu0zD5Rdg8uLE80J3ddRXnWYHCVf-31L_CHwhSXw at mail dot gmail dot com>
On Mon, 17 Aug 2015, H.J. Lu wrote:
> On Mon, Aug 17, 2015 at 12:35 PM, Alexander Monakov <amonakov@ispras.ru> wrote:
> > On Mon, 17 Aug 2015, H.J. Lu wrote:
> >
> >> On Mon, Aug 17, 2015 at 11:05 AM, Alexander Monakov <amonakov@ispras.ru> wrote:
> >> > On Mon, 17 Aug 2015, H.J. Lu wrote:
> >> >
> >> >> On Mon, Aug 17, 2015 at 10:53 AM, Alexander Monakov <amonakov@ispras.ru> wrote:
> >> >> >> > Earlier you rejected the idea that a resolver can use an externally visible
> >> >> >> > symbol. I just don't see why you say it might be non-deterministic. One
> >> >> >> > can inspect the executable, LD_PRELOAD'ed modules, etc. to see whether they
> >> >> >> > are going to provide an interposing definition of a symbol or not. What is
> >> >> >> > non-deterministic in there?
> >> >> >>
> >> >> >> Due to the way how IFUNC is implemented, dependency on
> >> >> >> external relocation may lead to undefined behavior. We tried
> >> >> >> to support external relocations in IFUNC selector:
> >> >> >>
> >> >> >> https://sourceware.org/bugzilla/show_bug.cgi?id=13302
> >> >> >>
> >> >> >> But external relocation on function address returned by IFUNC
> >> >> >> selector isn't supported.
> >> >> >
> >> >> > Isn't this an implementation deficiency that can be solved, for example, by
> >> >> > sorting GLOB_DAT relocation appropriately, rather than a fundamental
> >> >> > restriction that any reasonable implementation of IFUNC would exhibit?
> >> >>
> >> >> We have no control over GLOB_DAT, which can be a relocation
> >> >> against IFUNC symbol and we only know for sure at run-time.
> >> >
> >> > Surely such a counter-argument applies equally to JUMP_SLOT relocations?
> >> > Imagine that 'extern int f(void)' in bug 13302 that you've linked turns
> >> > out to be an IFUNC symbol at runtime.
> >> >
> >> > extern int f(void);
> >> >
> >> > void alt1(void) { }
> >> > void alt2(void) { }
> >> >
> >> > static void (*resolve (void)) (void)
> >> > {
> >> > return f() ? alt1 : alt2;
> >> > }
> >> >
> >> > void fct(void) __attribute__ ((ifunc ("resolve")));
> >> >
> >>
> >> The difference here is symbol definition for GLOB_DAT is known
> >> only at run-time while relocations are known at link-time.
> >
> > Sorry, I just don't see what point you're trying to make here.
> >
> > Anyway. Suppose 'resolve()' is compiled with -fno-plt. Then instead of
> > JUMP_SLOT relocation to 'f' it'll have a GLOB_DAT relocation to 'f', correct?
> > What's going to happen then?
> >
>
> I believe it is OK since GLOB_DAT relocations are performed before
> IRELATIVE relocations in .rela.plt / .rel.plt section.
OK. Now further suppose that we are
a) calling 'fct' with -fno-plt, or
b) taking the address of 'fct' (as in Ondrey's original example)
In both cases we're going to have a GLOB_DAT relocation to 'fct'.
So we have a GLOB_DAT relocation to 'fct', which is an IFUNC symbol with
'resolve' as its resolver, and the resolver invokes 'f', using a GLOB_DAT
relocation.
Are you saying it's not supposed to work?
Thanks.
Alexander