Re: [PATCH] perf: x86: fix callgraphs of 32-bit processes on 64-bit kernels V2.

[Ingo Molnar <mingo@elte.hu>]

* T??r??k Edwin <edwintorok@gmail.com> 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: T??r??k 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..b85ea9f 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;
> +};

Please put such new data type definitions not into the middle of a .c file but 
next to where struct stack_frame is defined.

> +
> +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);
> +}
>

Single-use - should be inline i guess.

> +
>  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) {

Please put newlines after local variable definition so that they are clearly 
delimited.

Also, the tabulation is weird - please run it through scripts/checkpatch.pl.

> +		frame.next_frame     = 0;
> +		frame.return_address = 0;
> +
> +		if (!copy_stack_frame_ia32(fp, &frame))
> +			break;
> +
> +		if (fp < (u32)regs->sp)
> +			break;
> +
> +		callchain_store(entry, frame.return_address);
> +		fp = frame.next_frame;
> +	    }
> +	    return;

This whole new block should probably be in a helper inline?

Also, it should probably be #ifdef CONFIG_COMPAT or so.

Thanks,

	Ingo