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RFC: Add DT_FLAGS_2 and DF_2_GNU_IFUNC

H.J. Lu hjl.tools@gmail.com
Fri May 25 22:09:00 GMT 2018 
On Fri, May 25, 2018 at 3:05 PM, Florian Weimer <fweimer@redhat.com> wrote:
> On 05/25/2018 11:58 PM, H.J. Lu wrote:
>>
>> On Fri, May 25, 2018 at 2:26 PM, Florian Weimer <fweimer@redhat.com>
>> wrote:
>>>
>>> On 05/25/2018 11:18 PM, H.J. Lu wrote:
>>>
>>>>> Still I don't see what this buys you if the LD_PRELOAD object has
>>>>> IFUNCs,
>>>>> too.
>>>>
>>>>
>>>>
>>>> [hjl@gnu-cfl-1 ifunc-2]$ cat bar.c
>>>> void
>>>> bar (char *dst, const char *src, unsigned int size)
>>>> {
>>>>     __builtin_memmove (dst, src, size);
>>>> }
>>>> [hjl@gnu-cfl-1 ifunc-2]$ cat main.c
>>>> extern void bar (char *, const char *, unsigned int);
>>>>
>>>> int
>>>> main ()
>>>> {
>>>>     char dst[50];
>>>>     const char src[] =
>>>>       {
>>>>         "This is a test"
>>>>       };
>>>>     bar (dst, src, sizeof (src));
>>>>     if (__builtin_memcmp (dst, src, sizeof (src)) != 0)
>>>>       __builtin_abort ();
>>>>     return 0;
>>>> }
>>>> [hjl@gnu-cfl-1 ifunc-2]$ make
>>>> gcc -O2 -g   -c -o main.o main.c
>>>> gcc -O2 -g -fPIC   -c -o bar.o bar.c
>>>> gcc -shared -z now -o libbar.so bar.o
>>>> gcc -z now -o x main.o libbar.so -Wl,-R,.
>>>> ./x
>>>> [hjl@gnu-cfl-1 ifunc-2]$ LD_PRELOAD=./libmemmove.so ./x
>>>> ./x: Relink `./libbar.so' with `./libmemmove.so' for IFUNC symbol
>>>> `memmove'
>>>> [hjl@gnu-cfl-1 ifunc-2]$
>>>
>>>
>>>
>>> This doesn't reproduce for me.  The commands above do not produce
>>> libmemmove.so.  “LD_PRELOAD=./libbar.so ./x” does not crash here (with a
>>> glibc 2.26-derived library that hasn't got my IFUNC scheduler).
>
>
> (some commands are missing from the reproduction steps)
>
>> https://sourceware.org/bugzilla/show_bug.cgi?id=23240
>
>
> This is just the old known issue related to symbol interposition and IFUNC
> resolvers, where even complete DT_NEEDED information is not sufficient to
> perform the relocations in the correct order.
>
> A single bit will not solve this.  In your reproducer, add an IFUNC resolver
> to libbar.so, and I'm sure the bug will resurface again.

Can you add a complete testcase to

https://sourceware.org/bugzilla/show_bug.cgi?id=23240


-- 
H.J.
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