[Bug c/66826] New: Unused dlsym return results in segfault

2015-07-09 Thread quae at daurnimator dot com 
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66826

Bug ID: 66826
   Summary: Unused dlsym return results in segfault
   Product: gcc
   Version: unknown
Status: UNCONFIRMED
  Severity: normal
  Priority: P3
 Component: c
  Assignee: unassigned at gcc dot gnu.org
  Reporter: quae at daurnimator dot com
  Target Milestone: ---

Having a weird issue here: if I dlsym() something in a constructor, but don't
use the result; the shared library segfaults at load.

bug-main.c:

#define _GNU_SOURCE
#include 

int main() {
void *foo = dlopen("./shared.so", RTLD_NOW);
void (*some_exported_function)() = dlsym(foo,
"some_exported_function");
some_exported_function();
return 0;
}

bug-shared.c:

#define _GNU_SOURCE
#include 
#include 

static int (*pointer) ();
static int stub () { return 0; }
__attribute__((constructor)) static void some_init() {
if ((pointer = dlsym(RTLD_DEFAULT, "anything")) == NULL) {
pointer = stub;
}
}

void some_exported_function() {
printf("in some_exported_function\n");
printf("%p\n", pointer);
}


Compile with:

gcc -shared -O2 -fPIC bug-shared.c -ldl -o shared.so
gcc -O2 -fPIC bug-main.c -ldl


Now comment out the `printf` of the pointer in bug-shared.c; and it'll get a
segfault.

Backtrace:
#0  0x77de4b77 in _dl_lookup_symbol_x () from
/lib64/ld-linux-x86-64.so.2
#1  0x77951b91 in do_sym () from /usr/lib/libc.so.6
#2  0x77bd80f4 in ?? () from /usr/lib/libdl.so.2
#3  0x77de9f94 in _dl_catch_error () from /lib64/ld-linux-x86-64.so.2
#4  0x77bd85e1 in ?? () from /usr/lib/libdl.so.2
#5  0x77bd8148 in dlsym () from /usr/lib/libdl.so.2
#6  0x77dea0ea in call_init.part () from /lib64/ld-linux-x86-64.so.2
#7  0x77dea1fb in _dl_init () from /lib64/ld-linux-x86-64.so.2
#8  0x77dee627 in dl_open_worker () from /lib64/ld-linux-x86-64.so.2
#9  0x77de9f94 in _dl_catch_error () from /lib64/ld-linux-x86-64.so.2
#10 0x77dede01 in _dl_open () from /lib64/ld-linux-x86-64.so.2
#11 0x77bd7fc9 in ?? () from /usr/lib/libdl.so.2
#12 0x77de9f94 in _dl_catch_error () from /lib64/ld-linux-x86-64.so.2
#13 0x77bd85e1 in ?? () from /usr/lib/libdl.so.2
#14 0x77bd8061 in dlopen () from /usr/lib/libdl.so.2
#15 0x00400622 in main ()

$ gcc -dumpversion
5.1.0

Works fine with -O0
Works fine with clang

[Bug c/66826] Unused result from dlsym in constructor results in a segfault

2015-07-10 Thread quae at daurnimator dot com 
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66826

--- Comment #1 from Daurnimator  ---
nalaginrut (from #gcc irc channel) confirmed that the issue only occurs in GCC
5 at -O2 and higher. (the bug is *not* present in GCC 4.9 or at -O1).

Note: only the compilation of shared.so matters (the main binary can be
compiled with any setting or compiler and the same behaviour is shown.)

[Bug c/66826] Unused result from dlsym in constructor results in a segfault

2015-07-12 Thread quae at daurnimator dot com 
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66826

--- Comment #2 from Daurnimator  ---
I recompiled glibc with debug symbols.

Program received signal SIGSEGV, Segmentation fault.
_dl_lookup_symbol_x (undef_name=0x77634775 "anything",
undef_map=0x77ffd998 <_rtld_local+2456>, ref=0x7fffdca8, 
symbol_scope=0x0, version=0x0, type_class=0, flags=3, skip_map=0x0) at
dl-lookup.c:769
769 while ((*scope)->r_list[i] != skip_map)
(gdb) bt
#0  _dl_lookup_symbol_x (undef_name=0x77634775 "anything",
undef_map=0x77ffd998 <_rtld_local+2456>, ref=0x7fffdca8, 
symbol_scope=0x0, version=0x0, type_class=0, flags=3, skip_map=0x0) at
dl-lookup.c:769
#1  0x77951b91 in do_sym (handle=0x0, name=0x77634775 "anything",
who=, vers=0x0, flags=2) at dl-sym.c:161
#2  0x77bd80f4 in ?? () from /usr/lib/libdl.so.2
#3  0x77de9f94 in _dl_catch_error (objname=0x77dda0f0,
errstring=0x77dda0f8, mallocedp=0x77dda0e8, 
operate=0x77bd80e0, args=0x7fffdea0) at dl-error.c:187
#4  0x77bd85e1 in ?? () from /usr/lib/libdl.so.2
#5  0x77bd8148 in dlsym () from /usr/lib/libdl.so.2
#6  0x77dea0ea in call_init (l=, argc=argc@entry=1,
argv=argv@entry=0x7fffe498, env=env@entry=0x7fffe4a8)
at dl-init.c:72
#7  0x77dea1fb in call_init (env=0x7fffe4a8, argv=0x7fffe498,
argc=1, l=) at dl-init.c:30
#8  _dl_init (main_map=main_map@entry=0x601030, argc=1, argv=0x7fffe498,
env=0x7fffe4a8) at dl-init.c:120
#9  0x77dee627 in dl_open_worker (a=a@entry=0x7fffe168) at
dl-open.c:568
#10 0x77de9f94 in _dl_catch_error
(objname=objname@entry=0x7fffe158,
errstring=errstring@entry=0x7fffe160, 
mallocedp=mallocedp@entry=0x7fffe157,
operate=operate@entry=0x77dee260 ,
args=args@entry=0x7fffe168)
at dl-error.c:187
#11 0x77dede01 in _dl_open (file=0x4006e4 "./shared.so",
mode=-2147483646, caller_dlopen=0x400535 , nsid=-2, argc=1, 
argv=, env=0x7fffe4a8) at dl-open.c:652
#12 0x77bd7fc9 in ?? () from /usr/lib/libdl.so.2
#13 0x77de9f94 in _dl_catch_error (objname=0x77dda0f0,
errstring=0x77dda0f8, mallocedp=0x77dda0e8, 
operate=0x77bd7f70, args=0x7fffe380) at dl-error.c:187
#14 0x77bd85e1 in ?? () from /usr/lib/libdl.so.2
#15 0x77bd8061 in dlopen () from /usr/lib/libdl.so.2
#16 0x00400535 in main ()
(gdb) info locals
old_hash = 4294967295
current_value = {s = 0x0, m = 0x0}
scope = 0x0
__PRETTY_FUNCTION__ = "_dl_lookup_symbol_x"
i = 0
protected = 
(gdb) info args
undef_name = 0x77634775 "anything"
undef_map = 0x77ffd998 <_rtld_local+2456>
ref = 0x7fffdca8
symbol_scope = 0x0
version = 0x0
type_class = 0
flags = 3
skip_map = 0x0