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[Qemu-devel] how do we determine correct guest PC for segfaults in atomi

From: Peter Maydell
Subject: [Qemu-devel] how do we determine correct guest PC for segfaults in atomic helpers for linux-user mode?
Date: Mon, 13 Nov 2017 19:59:50 +0000

I've been investigating a bug (a javac crash). I'm not sure if it's
the root cause, but I can't figure out how, if we get a guest SEGV in
an atomic helper we report the right faulting PC to the guest.

Specifically, if you get a SEGV here:

#0  0x000000006003c22b in helper_atomic_cmpxchgl_le (env=0x63caf680,
    addr=275041819628, cmpv=0, newv=1)
#1  0x0000000061002f61 in static_code_gen_buffer ()
#2  0x0000000060035d6b in cpu_tb_exec (cpu=0x63ca73e0,
    itb=0x6119d000 <static_code_gen_buffer+9080960>)
    at /home/petmay01/linaro/qemu-from-laptop/qemu/accel/tcg/cpu-exec.c:167
#3  0x0000000060036945 in cpu_loop_exec_tb (cpu=0x63ca73e0,
    tb=0x6119d000 <static_code_gen_buffer+9080960>, last_tb=0x7f01b213dbd8,
    at /home/petmay01/linaro/qemu-from-laptop/qemu/accel/tcg/cpu-exec.c:611
#4  0x0000000060036bc2 in cpu_exec (cpu=0x63ca73e0)
    at /home/petmay01/linaro/qemu-from-laptop/qemu/accel/tcg/cpu-exec.c:723
#5  0x000000006003da13 in cpu_loop (env=0x63caf680)
    at /home/petmay01/linaro/qemu-from-laptop/qemu/linux-user/main.c:809
#6  0x000000006004c627 in clone_func (arg=0x7ffe028f0a10)
    at /home/petmay01/linaro/qemu-from-laptop/qemu/linux-user/syscall.c:6241
#7  0x00000000602fcc25 in start_thread (arg=0x7f01b213e700)
    at pthread_create.c:333
#8  0x00000000603949a9 in clone ()

then the code in handle_cpu_signal() is passed a pc of 0x6003c22b
(the location in the helper function that does the memory access).
This is outside generated code, so the call to cpu_restore_state()
in handle_cpu_signal() will do nothing. However as far as I can tell,
there isn't any syncing of the PC etc state to the CPU before calling
this helper (at least, env->pc is completely wrong for the insn that
I think is causing this helper call).

Am I misreading my debugger entrails (entirely possible)? How is this
code intended to get the right guest PC for segfaults in these helpers?

(I'll investigate further tomorrow, but since it's end-of-day for me
I figured I'd throw this question out before going home...)

-- PMM

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