Re: [Bug] DWARF-5 section names in PE/PEP and weak symbols

[Achim Gratz <Stromeko@nexgo.de>]

So here's another attempt at getting to the bottom of this.  The failing
gnulib test was using fputs, the actually failing gnulib module was
calling thrd_exit if the weak symbol resolved non-NULL and
serendipitously I've stumbled upon snprintf behaving differently…

--8<---------------cut here---------------start------------->8---
#include <stdio.h>
#include <pthread.h>
#include <threads.h>
#pragma weak fputs
#pragma weak thrd_exit
#pragma weak snprintf
#pragma weak asnprintf
#pragma weak vsnprintf
int main ()
{
  char buf[256] = "snprintf was not called.\n";
  printf( "snprintf  => %014p\tis %sequal NULL\n", snprintf,  ((snprintf  == NULL) ? "" : "not ") );
  printf( "fputs     => %014p\tis %sequal NULL\n", fputs,     ((fputs     == NULL) ? "" : "not ") );
  printf( "thrd_exit => %014p\tis %sequal NULL\n", thrd_exit, ((thrd_exit == NULL) ? "" : "not ") );
  if (snprintf != NULL)
    snprintf(buf, sizeof(buf), "%s\n", "sprintf called through symbol");
  printf( "%s", buf );
  return (fputs == NULL);
}
--8<---------------cut here---------------end--------------->8---

This is what happens:

--8<---------------cut here---------------start------------->8---
...packages/binutils-gdb/build (2130)# gcc -Wl,--trace-symbol={fputs,snprintf,thrd_exit} -c -o weak_test.o weak_test.c
...packages/binutils-gdb/build (2131)# gcc -Wl,--trace-symbol={fputs,{a,v,}snprintf,thrd_exit} -o weak_test weak_test.o
/usr/lib/gcc/x86_64-pc-cygwin/11/../../../../x86_64-pc-cygwin/bin/ld: weak_test.o: reference to snprintf
/usr/lib/gcc/x86_64-pc-cygwin/11/../../../../x86_64-pc-cygwin/bin/ld: weak_test.o: reference to thrd_exit
/usr/lib/gcc/x86_64-pc-cygwin/11/../../../../x86_64-pc-cygwin/bin/ld: weak_test.o: reference to fputs
/usr/lib/gcc/x86_64-pc-cygwin/11/../../../../x86_64-pc-cygwin/bin/ld: /usr/lib/gcc/x86_64-pc-cygwin/11/../../../../lib/libcygwin.a(t-d000578.o): definition of fputs
/usr/lib/gcc/x86_64-pc-cygwin/11/../../../../x86_64-pc-cygwin/bin/ld: /usr/lib/gcc/x86_64-pc-cygwin/11/../../../../lib/libcygwin.a(t-d001527.o): definition of thrd_exit
...packages/binutils-gdb/build (2132)# ./weak_test ; echo $?
snprintf  => 0x000000000000     is equal NULL
fputs     => 0x0001004012e0     is not equal NULL
thrd_exit => 0x000100401300     is not equal NULL
snprintf was not called.
0
...packages/binutils-gdb/build (2133)# nm weak_test.o /bin/cygwin1.dll | egrep ' (fputs|[av]?snprintf|thrd_exit)$'
                 U fputs
                 w snprintf
                 U thrd_exit
00000001801abb10 T asnprintf
00000001801b1610 T fputs
00000001801c0380 T snprintf
00000001801849a0 T thrd_exit
00000001801eec30 T vsnprintf
--8<---------------cut here---------------end--------------->8---

So, snprintf seems to behave as expected (it resolves NULL) and the two
other symbols are resolved even though they should not.  If they had
resolved to the actual address things might even work, but the actual
values explain nicely why a program calling through the pointer crashes
in various horrible ways (depending on what actually is mapped there).


Regards,
Achim.
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