Re: [!!Mass Mail]Strange problem which involves libpthread and link flag.

[Sergey Gvozdetskiy <gvozdetskiy at corp dot sputnik dot ru>]

Hello, Goffredo, and everybody!
this bug doesn't happen with libc-2.21 and gcc version 4.9.3 too.

My strace of this one example:
$ LD_LIBRARY_PATH=$(pwd) strace ./boom
execve("./boom", ["./boom"], [/* 91 vars */]) = 0
brk(0)                                  = 0x1f39000
access("/etc/ld.so.nohwcap", F_OK)      = -1 ENOENT (No such file or 
directory)
mmap(NULL, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) 
= 0x7ff1e40f4000
access("/etc/ld.so.preload", R_OK)      = -1 ENOENT (No such file or 
directory)
open("/home/gvozdetskiy/tmp/src/tls/x86_64/libdofork.so", 
O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory)
stat("/home/gvozdetskiy/tmp/src/tls/x86_64", 0x7fffde6a46d0) = -1 ENOENT 
(No such file or directory)
open("/home/gvozdetskiy/tmp/src/tls/libdofork.so", O_RDONLY|O_CLOEXEC) = 
-1 ENOENT (No such file or directory)
stat("/home/gvozdetskiy/tmp/src/tls", 0x7fffde6a46d0) = -1 ENOENT (No 
such file or directory)
open("/home/gvozdetskiy/tmp/src/x86_64/libdofork.so", 
O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory)
stat("/home/gvozdetskiy/tmp/src/x86_64", 0x7fffde6a46d0) = -1 ENOENT (No 
such file or directory)
open("/home/gvozdetskiy/tmp/src/libdofork.so", O_RDONLY|O_CLOEXEC) = 3
read(3, 
"\177ELF\2\1\1\0\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\320\5\0\0\0\0\0\0"..., 
832) = 832
fstat(3, {st_mode=S_IFREG|0775, st_size=7760, ...}) = 0
mmap(NULL, 2101264, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 
0) = 0x7ff1e3cd1000
mprotect(0x7ff1e3cd2000, 2093056, PROT_NONE) = 0
mmap(0x7ff1e3ed1000, 8192, PROT_READ|PROT_WRITE, 
MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0) = 0x7ff1e3ed1000
close(3)                                = 0
open("/home/gvozdetskiy/tmp/src/libc.so.6", O_RDONLY|O_CLOEXEC) = -1 
ENOENT (No such file or directory)
open("/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=118554, ...}) = 0
mmap(NULL, 118554, PROT_READ, MAP_PRIVATE, 3, 0) = 0x7ff1e40d7000
close(3)                                = 0
access("/etc/ld.so.nohwcap", F_OK)      = -1 ENOENT (No such file or 
directory)
open("/lib/x86_64-linux-gnu/libc.so.6", O_RDONLY|O_CLOEXEC) = 3
read(3, 
"\177ELF\2\1\1\3\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0`\v\2\0\0\0\0\0"..., 832) 
= 832
fstat(3, {st_mode=S_IFREG|0755, st_size=1869392, ...}) = 0
mmap(NULL, 3972864, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 
0) = 0x7ff1e3907000
mprotect(0x7ff1e3ac7000, 2097152, PROT_NONE) = 0
mmap(0x7ff1e3cc7000, 24576, PROT_READ|PROT_WRITE, 
MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x1c0000) = 0x7ff1e3cc7000
mmap(0x7ff1e3ccd000, 16128, PROT_READ|PROT_WRITE, 
MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x7ff1e3ccd000
close(3)                                = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) 
= 0x7ff1e40d6000
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) 
= 0x7ff1e40d5000
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) 
= 0x7ff1e40d4000
arch_prctl(ARCH_SET_FS, 0x7ff1e40d5700) = 0
mprotect(0x7ff1e3cc7000, 16384, PROT_READ) = 0
mprotect(0x7ff1e3ed1000, 4096, PROT_READ) = 0
mprotect(0x600000, 4096, PROT_READ)     = 0
mprotect(0x7ff1e40f6000, 4096, PROT_READ) = 0
munmap(0x7ff1e40d7000, 118554)          = 0
clone(child_stack=0, 
flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|SIGCHLD, 
child_tidptr=0x7ff1e40d59d0) = 24024
exit_group(24024)                       = ?
+++ exited with 216 +++

Return value after execute ./boom was 36, 40, 43, etc.

Same strace out(with minimal differents) I had, while use gcc version 
5.2.1.
Return value of it, was 0.

Ubuntu 15.10, gcc's from repo.

It for more details, may be helpful.))
Best regards!

On 03/17/2016 01:50 AM, 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)
>
>
> $ dpkg --list | grep libc6
> ii  libc6:amd64                                   2.22-3
>
> I was able to reproduce this bug also in a Fedora F23 machine (=libc 2.22). The bug happens even with different compiler versions (gcc-4.8, gcc-6, gcc-5.3, clang-3.8). In another regular debian machine equipped with libc-2.19 the crash doesn't happen. All the machines tested are linux x86-64.
>
> In order to reproduce this problem:
> - use the "-Wl,-z,now" in the shared library
> - put fork() in the shared library
> - compile the main program with "-lpthread" before the "-l<shared library>" (the order matters !)
>
> So my questions are:
> - I am wrong to suppose that this code should not crash ?
> - I know that -lpthread is not needed, but is it forbid to use it even if not needed ? With -pthread only, the code works without problem (mosh too !)
> - Could the problem be libc/pthread related ?
>
>
> BR
> G.Baroncelli
>
> [*] https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=817929
> mosh problem