On Fri, 2018-09-14 at 03:40:04 UTC, Alan Modra wrote: > Call Frame Information is used by gdb for back-traces and inserting > breakpoints on function return for the "finish" command. This failed > when inside __kernel_clock_gettime. More concerning than difficulty > debugging is that CFI is also used by stack frame unwinding code to > implement exceptions. If you have an app that needs to handle > asynchronous exceptions for some reason, and you are unlucky enough to > get one inside the VDSO time functions, your app will crash. > > What's wrong: There is control flow in __kernel_clock_gettime that > reaches label 99 without saving lr in r12. CFI info however is > interpreted by the unwinder without reference to control flow: It's a > simple matter of "Execute all the CFI opcodes up to the current > address". That means the unwinder thinks r12 contains the return > address at label 99. Disabuse it of that notion by resetting CFI for > the return address at label 99. > > Note that the ".cfi_restore lr" could have gone anywhere from the > "mtlr r12" a few instructions earlier to the instruction at label 99. > I put the CFI as late as possible, because in general that's best > practice (and if possible grouped with other CFI in order to reduce > the number of CFI opcodes executed when unwinding). Using r12 as the > return address is perfectly fine after the "mtlr r12" since r12 on > that code path still contains the return address. > > __get_datapage also has a CFI error. That function temporarily saves > lr in r0, and reflects that fact with ".cfi_register lr,r0". A later > use of r0 means the CFI at that point isn't correct, as r0 no longer > contains the return address. Fix that too. > > Signed-off-by: Alan Modra <amo...@gmail.com> > Tested-by: Reza Arbab <ar...@linux.ibm.com>
Applied to powerpc next, thanks. https://git.kernel.org/powerpc/c/56d20861c027498b5a1112b4f9f05b cheers
