Re: Strange problem which involves libpthread and link flag.

[Goffredo Baroncelli <kreijack at inwind dot it>]

Hi,
I filed the bug 19861

https://sourceware.org/bugzilla/show_bug.cgi?id=19861

BR
G.Baroncelli


On 2016-03-23 17:18, Florian Weimer wrote:
> On 03/16/2016 11:50 PM, Goffredo Baroncelli wrote:
>> Hi All,
>>
>> I hope that this is the right place where post this kind of question. If this is not the case, sorry for the inconvenience and please give me a suggestion where I have to put this question.
>>
>> Investigating the reason why mosh crashed on my debian machine [*], I was able to create a test case to reproduce the crash.
>>
>> I have to point out that the problem which I found was not related to mosh, but (I suppose) to a strange interaction between some linker flag and the using of the pthread library.
>>
>>
>> $ cat boom.c
>> extern void dofork();
>>
>> int main() {
>>      dofork();
>> }
>>
>> $ cat dofork.c
>> #include <unistd.h>
>>
>> void dofork() {
>> 	fork();
>> }
>>
>> $ gcc -fPIC -c dofork.c
>> $ gcc -shared -Wl,-z,now -o libdofork.so dofork.o
>> $ gcc -o boom boom.c -lpthread -L$(pwd) -ldofork
>> $ LD_LIBRARY_PATH=$(pwd) ./boom
>> Segmentation fault
>>
>> $ LD_LIBRARY_PATH=$(pwd) ldd ./boom linux-vdso.so.1 (0x00007ffe817dc000)
>> libpthread.so.0 => /lib/x86_64-linux-gnu/libpthread.so.0 (0x00007f16b38ed000)
>> libdofork.so => /home/ghigo/mosh/libdofork.so (0x00007f16b36ec000)
>> libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f16b3347000)
>> /lib64/ld-linux-x86-64.so.2 (0x0000562eba12a000)
> 
> Hi Goffredo,
> 
> this is a bug.  Could you report it in Bugzilla here, please?
> 
>    https://sourceware.org/bugzilla/enter_bug.cgi?product=glibc&component=nptl
> 
> I gathered the following additional information:
> 
> (gdb) break dofork
> Breakpoint 1 at 0x4005b0
> (gdb) r
> Starting program: /home/fweimer/boom
> [Thread debugging using libthread_db enabled]
> Using host libthread_db library "/lib64/libthread_db.so.1".
> 
> Breakpoint 1, 0x00007ffff79bd6d4 in dofork () from
> /home/fweimer/libdofork.so
> (gdb) disassemble
> Dump of assembler code for function dofork:
>     0x00007ffff79bd6d0 <+0>:     push   %rbp
>     0x00007ffff79bd6d1 <+1>:     mov    %rsp,%rbp
> => 0x00007ffff79bd6d4 <+4>:     callq  0x7ffff79bd5c0 <fork@plt>
>     0x00007ffff79bd6d9 <+9>:     nop
>     0x00007ffff79bd6da <+10>:    pop    %rbp
>     0x00007ffff79bd6db <+11>:    retq
> End of assembler dump.
> (gdb) si
> 0x00007ffff79bd5c0 in fork@plt () from /home/fweimer/libdofork.so
> (gdb) disassemble
> Dump of assembler code for function fork@plt:
> => 0x00007ffff79bd5c0 <+0>:     jmpq   *0x200a0a(%rip)        #
> 0x7ffff7bbdfd0 <fork@got.plt>
>     0x00007ffff79bd5c6 <+6>:     pushq  $0x2
>     0x00007ffff79bd5cb <+11>:    jmpq   0x7ffff79bd590
> End of assembler dump.
> (gdb) print *(void **)0x7ffff7bbdfd0
> $1 = (void *) 0x0
> (gdb)
> 
> The commit you identified, beff1d132c16aedd87a3f1bc7b572c8e69819015,
> assumes that __libc_fork has been relocated before the IFUNC resolver
> for the libpthread fork definition runs, which is not always true.
> 
> Florian
> 


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