Ok, I was able to track down a __very__ minimal test case: the problem seems to
be a "strange" interaction between the linker flag used during the creation of
libutemper, and the inclusion of -lpthread[*] during the linking of mosh-server.
It is not a problem related to mosh and/or libutempter. In fact I was able to
create a test without involving these two.
$ cat boom.c
extern void dofork();
int main() {
dofork();
}
$ cat dofork.c
#include <unistd.h>
void dofork() {
fork();
}
$ gcc -shared -Wl,-z,now -o libdofork.so dofork.o
$ gcc -o boom boom.c -lpthread -L$(pwd) -ldofork
$ 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)
$ LD_LIBRARY_PATH=$(pwd) ./boom
Segmentation fault
The key points are:
- put a fork in a shared library
- link the shared library with "-Wl,-z,now"
- compile a program with libpthread and the library above (note: libpthread is
not needed)
- put libpthread before the library above
Then I got a crash. The test was performed on a debian machine with libc6 2.22.
Even on a fedora F23 (=libc-2.22) I got the crash.
If I performed the same test on a debian machine with libc6 2.19 instead of
libc6 2.22, the crash doesn't happen.
If I remove "-lpthread" or "-Wl,-z,now" flag, the crash doesn't happen.
If I put the fork() in the main(), the crash doesn't happen.
My (limitated) knowledge doesn't suggest me any valid reason about why the
crashes. However
- if I remove "-Wl,-z,now" from libutempter mosh-server doesn't crash.
- if I remove -lpthread from the linking of "mosh-server", mosh doesn't crash
anymore. Pay attention that -lpthread is not needed, because the -pthread flag
is sufficient.
BR
G.Baroncelli
[*] BTW -lpthread was required by libprotobuf
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