Arnd Bergmann arnd@arndb.de wrote on 08/26/2011 04:44:26 PM:
On Thursday 25 August 2011, Russell King - ARM Linux wrote:
Arnd, can you test this to make sure your gdb test case still works,
and
Mark, can you test this to make sure it fixes your problem please?
Hi Russell,
The patch in question was not actually from me but from Ulrich Weigand, so he's probably the right person to test your patch. I'm forwarding it in full to Uli for reference.
Hi Arnd, hi Russell,
sorry for the late reply, I've just returned from vacation today ...
I've not yet run the test, but just from reading through the patch it seems that this will at least partially re-introduce the problem my original patch was trying to fix.
The situation here is about GDB performing an "inferior function call", e.g. via the GDB "call" command. To do so, GDB will:
0. [ Have gotten control of the target process via some ptrace intercept previously, and then ... ] 1. Save the register state 2. Create a dummy frame on the stack and set up registers (PC, SP, argument registers, ...) as appropriate for a function call 3. Restart via PTRACE_CONTINUE [ ... at this point, the target process runs the function until it returns to a breakpoint instruction and GDB gets control again via another ptrace intercept ... ] 4. Restore the register state saved in [1.] 5. At some later point, continue the target process [at its original location] with PTRACE_CONTINUE
The problem now occurs if at point [0.] the target process just happened to be blocked in a restartable system call. For this sequence to then work as expected, two things have to happen:
- at point [3.], the kernel must *not* attempt to restart a system call, even though it thinks we're stopped in a restartable system call
- at point [5.], the kernel now *must* restart the originally interrupted system call, even though it thinks we're stopped at some breakpoint, and not within a system call
My patch achieved both these goals, while it would seem your patch only solves the first issue, not the second one. In fact, since any interaction with ptrace will always cause the TIF_SYS_RESTART flag to be *reset*, and there is no way at all to *set* it, there doesn't appear to be any way for GDB to achive that second goal.
[ With my patch, that second goal was implicitly achieved by the fact that at [1.] GDB would save a register state that already corresponds to the way things should be for restarting the system call. When that register set is then restored in [4.], restart just happens automatically without any further kernel intervention. ]
One way to fix this might be to make the TIF_SYS_RESTART flag itself visible to ptrace, so the GDB could save/restore it along with the rest of the register set; this would be similar to how that problem is handled on other platforms. However, there doesn't appear to be an obvious place for the flag in the ptrace register set ...
Bye, Ulrich