[Bug ld/19965] Copy relocation leads to change in read-only data

[amodra at gmail dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=19965

Alan Modra <amodra at gmail dot com> changed:

           What    |Removed                     |Added
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                 CC|                            |amodra at gmail dot com

--- Comment #3 from Alan Modra <amodra at gmail dot com> ---
Here's a testcase that I think does not invoke any undefined behaviour, and
shows various inconsistencies with protected variables and copy relocs. 
Depending on the version of gcc (and glibc!) you may see &x != x or no
segfault.
Compile protmain with -fPIC to see correct behaviour.

cat > prot.h <<\EOF
extern void *x;
extern void aprint (void);
extern void boom (void);
EOF
cat > prota.c <<\EOF
#include "prot.h"
void __attribute__ ((visibility ("protected"), section(".data.rel.ro")))
  *x = &x;
void aprint (void) { __builtin_printf ("A: &x = %p, x = %p\n", &x, x); }
EOF
cat > protb.c <<\EOF
#include "prot.h"
void boom (void) { x = 0; }
EOF
cat > protmain.c <<\EOF
#include "prot.h"
int
main (void)
{
  __builtin_printf ("main: &x = %p, x = %p\n", &x, x);
  aprint ();
  boom ();
  return 0;
}
EOF
gcc -O2 -fPIC -shared -o liba.so prota.c protb.c -Wl,-z,relro
gcc -O2 -o protmain protmain.c -L. -la -Wl,-rpath,.
./protmain

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