Re: [PATCH 2/2] Add an x86 IFUNC testcase for [BZ #20019]

[Khem Raj <raj dot khem at gmail dot com>]

On Fri, Jan 20, 2017 at 9:00 AM, H.J. Lu <hjl.tools@gmail.com> wrote:
> On Thu, Jan 19, 2017 at 10:42 AM, Khem Raj <raj.khem@gmail.com> wrote:
>> On Fri, Jan 13, 2017 at 12:15 PM, H.J. Lu <hjl.tools@gmail.com> wrote:
>>> On Fri, Jan 13, 2017 at 11:30 AM, Khem Raj <raj.khem@gmail.com> wrote:
>>>>
>>>>
>>>> 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.
>>>>
>>>
>>> Here is the updated patch with some fixes.
>>
>> This still failed on 32bit in same way.
>>
>
> Did you get segfault?

No, I was testing it for https://sourceware.org/bugzilla/show_bug.cgi?id=21041
I am getting same ldso IFUNC messages