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Re: [PATCH 2/2] Add an x86 IFUNC testcase for [BZ #20019]
- From: Khem Raj <raj dot khem at gmail dot com>
- To: libc-alpha at sourceware dot org
- Date: Fri, 13 Jan 2017 11:30:39 -0800
- Subject: Re: [PATCH 2/2] Add an x86 IFUNC testcase for [BZ #20019]
- Authentication-results: sourceware.org; auth=none
- References: <20161004184621.GB27454@intel.com> <alpine.DEB.2.20.1610042122170.17011@digraph.polyomino.org.uk> <CAMe9rOrvz5nZZUVMq5qc_FT2g9iKzpGQ6-ydESmmm3CP5BVmJg@mail.gmail.com> <alpine.DEB.2.20.1610042253170.17011@digraph.polyomino.org.uk> <CAMe9rOpzKV4oLgCrX+scC=MVH-TM_TRAcOZrLUg06L0YvEZuRw@mail.gmail.com> <alpine.DEB.2.20.1610050009080.17011@digraph.polyomino.org.uk> <CAMe9rOrtbOXOKYa2fiazJZb0WfZ0KbRr0thwvYOTdL=D3qHt-g@mail.gmail.com> <bc2016fe-c194-582b-dc80-a6392d51aa17@redhat.com> <CAMe9rOr=LKK6XVHGh2uehYgkRhLmEXMcEsdP25zd0ny8GPGfBA@mail.gmail.com> <CAMe9rOpEYGGw3MxO9v+J0u3WWN1A-Nm8p9=J1m7_7g9SA4e3pg@mail.gmail.com>
On 1/13/17 11:03 AM, H.J. Lu wrote:
> On Wed, Oct 12, 2016 at 3:19 PM, H.J. Lu <hjl.tools@gmail.com> wrote:
>> On Tue, Oct 11, 2016 at 10:45 PM, Carlos O'Donell <carlos@redhat.com> wrote:
>>> On 10/05/2016 02:16 PM, H.J. Lu wrote:
>>>> On Tue, Oct 4, 2016 at 5:09 PM, Joseph Myers <joseph@codesourcery.com> wrote:
>>>>> On Wed, 5 Oct 2016, H.J. Lu wrote:
>>>>>
>>>>>> I can try __builtin_memcpy, instread of __builtin_memmove. There are 2
>>>>
>>>> I changed it to use __builtin_memset.
>>>>
>>>>>> acceptable results. One is ld.so issues an error and the other is program runs.
>>>>>> On x86, ld.so issues an error. I don't know what should happen on others.
>>>>>
>>>>> You could make the test pass on either of those results (while failing if
>>>>> ld.so crashes).
>>>>>
>>>>
>>>> I moved the test to elf. It passes if the test runs or ld.so issues an
>>>> error. Please try it on arm, powerpc and s390.
>>>
>>> This is the wrong way to test this.
>>>
>>> The point of this test is this:
>>>
>>> - Verify that an unversioned symbol reference in DSO A which has no DT_NEEDED
>>> on DSO B, when resolved to a symbol definition in DSO B, when the symbol in
>>> DSO B is an IFUNC with a resolver, that DSO B is relocated _before_ the IFUNC
>>> resolver is called, because DSO B's resolver might need global data to make
>>> the IFUNC decision e.g. GOT setup.
>>>
>>> The invariant we want to hold true for IFUNC is that to call the resolver
>>> function you must have relocated the DSO which contains the resolver. This _should_
>>> have been done by a symbol reocation dependency analysis, but that isn't working
>>> correctly IMO or needs deeper analysis in the dynamic loader.
>>>
>>> The solution we want in place today is to issue some kind of diagnostic until we
>>> fix the real problem.
>>>
>>> The test should look like this:
>>>
>>> - DSO A with an unversioned symbol reference to 'foo'.
>>> - DSO B with a symbol definition of 'foo' as an ifunc with 'foo_resolver' as the
>>> resolver function which references global data from DSO C to decide which of
>>> two functions to return.
>>> - DSO C with global data set to a value.
>>>
>>> The point is that DSO B depends on DSO C and has DT_NEEDED on it, so C will get
>>> relocated first, then B, such that B's GOT is setup to access C's global data.
>>>
>>> When handling the reference to 'foo' in DSO A we should on x86_64 and i686
>>> get the error about needing to relink DSO A so it depends on DSO B, to form
>>> the initialization order of C->B->A.
>>>
>>> I expect this test case will now crash the other arches, rather than just
>>> avoiding the crash by relying on internal libc.so details about which ifuncs
>>> you're using.
>>>
>>> This is one step towards a better definition of IFUNC semantics, which need to
>>> be more clearly defined (something I wish I had time to define and fix so
>>> more projects could use them).
>>
>> IFUNC resolver can fail for various reasons. My goal is to make sure
>> that IFUNC inside of glibc works correctly or an error message is given
>> when glibc isn't used properly. In case of x86, CPU feature info is
>> retrieved and stored in ld.so very early at startup, which is used by IFUNC
>> and only accessible in libc.so and libm.so after they have been relocated.
>> My change in x86 ld.so checks it and my test verifies the check. My fix
>> won't cover other possible IFUNC failures.
>>
>
> When the IFUNC relocation is performed before the providing shared
> library is unrelocated, the returned function address will be 0 and
> program will segfault when the function is called.
>
> Please apply this patch and run the test if your platform has IFUNC. I only
> enabled the unsafe resolver check for i386 and x86-64. It is straightforward
> to add check for other platforms.
I will test it out shortly. One thing I see, the runner script for test
is calling out for /bin/bash and the script does not use any bash
extentions perhaps using /bin/sh is enough.
>
>
> H.J.
>