John David Anglin 1a044511a3 Fix data race in setting function descriptors during lazy binding on hppa. ... This addresses an issue that is present mainly on SMP machines running threaded code. In a typical indirect call or PLT import stub, the target address is loaded first. Then the global pointer is loaded into the PIC register in the delay slot of a branch to the target address. During lazy binding, the target address is a trampoline which transfers to _dl_runtime_resolve(). _dl_runtime_resolve() uses the relocation offset stored in the global pointer and the linkage map stored in the trampoline to find the relocation. Then, the function descriptor is updated. In a multi-threaded application, it is possible for the global pointer to be updated between the load of the target address and the global pointer. When this happens, the relocation offset has been replaced by the new global pointer. The function pointer has probably been updated as well but there is no way to find the address of the function descriptor and to transfer to the target. So, _dl_runtime_resolve() typically crashes. HP-UX addressed this problem by adding an extra pc-relative branch to the trampoline. The descriptor is initially setup to point to the branch. The branch then transfers to the trampoline. This allowed the trampoline code to figure out which descriptor was being used without any modification to user code. I didn't use this approach as it is more complex and changes function pointer canonicalization. The order of loading the target address and global pointer in indirect calls was not consistent with the order used in import stubs. In particular, $$dyncall and some inline versions of it loaded the global pointer first. This was inconsistent with the global pointer being updated first in dl-machine.h. Assuming the accesses are ordered, we want elf_machine_fixup_plt() to store the global pointer first and calls to load it last. Then, the global pointer will be correct when the target function is entered. However, just to make things more fun, HP added support for out-of-order execution of accesses in PA 2.0. The accesses used by calls are weakly ordered. So, it's possibly under some circumstances that a function might be entered with the wrong global pointer. However, HP uses weakly ordered accesses in 64-bit HP-UX, so I assume that loading the global pointer in the delay slot of the branch must work consistently. The basic fix for the race is a combination of modifying user code to preserve the address of the function descriptor in register %r22 and setting the least-significant bit in the relocation offset. The latter was suggested by Carlos as a way to distinguish relocation offsets from global pointer values. Conventionally, %r22 is used as the address of the function descriptor in calls to $$dyncall. So, it wasn't hard to preserve the address in %r22. I have updated gcc trunk and gcc-9 branch to not clobber %r22 in $$dyncall and inline indirect calls. I have also modified the import stubs in binutils trunk and the 2.33 branch to preserve %r22. This required making the stubs one instruction longer but we save one relocation. I also modified binutils to align the .plt section on a 8-byte boundary. This allows descriptors to be updated atomically with a floting-point store. With these changes, _dl_runtime_resolve() can fallback to an alternate mechanism to find the relocation offset when it has been clobbered. There's just one additional instruction in the fast path. I tested the fallback function, _dl_fix_reloc_arg(), by changing the branch to always use the fallback. Old code still runs as it did before. Fixes bug 23296. Reviewed-by: Carlos O'Donell <carlos@redhat.com>		2020-03-30 20:36:49 +00:00
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linux	Fix data race in setting function descriptors during lazy binding on hppa.	2020-03-30 20:36:49 +00:00