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    SubjectRe: [PATCH] perf: x86: fix callgraphs of 32-bit processes on 64-bit kernels.
    On 03/16/2010 04:49 PM, Frederic Weisbecker wrote:
    > On Mon, Mar 15, 2010 at 05:34:20PM +0200, Török Edwin wrote:
    >> When profiling a 32-bit process on a 64-bit kernel, callgraph tracing
    >> stopped after the first function, because it has seen a garbage memory address
    >> (tried to interpret the frame pointer, and return address as a 64-bit pointer).
    >>
    >> Fix this by using a struct stack_frame with 32-bit pointers when the TIF_IA32 flag is set.
    >>
    >> Note that TIF_IA32 flag must be used, and not is_compat_task(), because the
    >> latter is only set when the 32-bit process is executing a syscall,
    >> which may not always be the case (when tracing page fault events for example).
    >>
    >> Signed-off-by: Török Edwin <edwintorok@gmail.com>
    >> ---
    >> arch/x86/kernel/cpu/perf_event.c | 33 +++++++++++++++++++++++++++++++++
    >> 1 files changed, 33 insertions(+), 0 deletions(-)
    >>
    >> diff --git a/arch/x86/kernel/cpu/perf_event.c b/arch/x86/kernel/cpu/perf_event.c
    >> index 8c1c070..13ee83a 100644
    >> --- a/arch/x86/kernel/cpu/perf_event.c
    >> +++ b/arch/x86/kernel/cpu/perf_event.c
    >> @@ -2401,6 +2401,20 @@ static int copy_stack_frame(const void __user *fp, struct stack_frame *frame)
    >> return bytes == sizeof(*frame);
    >> }
    >>
    >> +struct stack_frame_ia32 {
    >> + u32 next_frame;
    >> + u32 return_address;
    >> +};
    >> +
    >> +static int copy_stack_frame_ia32(u32 fp, struct stack_frame_ia32 *frame)
    >> +{
    >> + unsigned long bytes;
    >> +
    >> + bytes = copy_from_user_nmi(frame, (const void __user*)(unsigned long)fp, sizeof(*frame));
    >> +
    >> + return bytes == sizeof(*frame);
    >> +}
    >> +
    >> static void
    >> perf_callchain_user(struct pt_regs *regs, struct perf_callchain_entry *entry)
    >> {
    >> @@ -2414,6 +2428,25 @@ perf_callchain_user(struct pt_regs *regs, struct perf_callchain_entry *entry)
    >>
    >> callchain_store(entry, PERF_CONTEXT_USER);
    >> callchain_store(entry, regs->ip);
    >> + if (test_thread_flag(TIF_IA32)) {
    >> + /* 32-bit process in 64-bit kernel. */
    >> + u32 fp = regs->bp;
    >> + struct stack_frame_ia32 frame;
    >> + while (entry->nr < PERF_MAX_STACK_DEPTH) {
    >> + frame.next_frame = 0;
    >> + frame.return_address = 0;
    >> +
    >> + if (!copy_stack_frame_ia32(fp, &frame))
    >> + break;
    >> +
    >> + if ((unsigned long)fp < regs->sp)
    >> + break;
    >
    >
    >
    > Shouldn't it be this?
    >
    > if (fp < (u32)regs->sp)
    >
    > As the high part of fp is zeroed in the cast but
    > the high part of regs->sp remains. I don't know what could be
    > there, but since the user doesn't use it, perhaps just garbage?
    > And that could messup the comparison.

    Agreed. I sent a new patch with that change.

    >
    > Otherwise the rest looks pretty good to me, nice catch.
    >

    Thanks.

    Best regards,
    --Edwin
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