Re: ARM builtin perf tests for breakpoint failures

[Will Deacon]

Hi Florian,

On Wed, Nov 14, 2018 at 12:21:12PM -0800, Florian Fainelli wrote:
> I have been trying to debug some perf builtin tests on ARM 32-bit and
> found that "Breakpoint overflow signal handler" and "Breakpoint overflow
> sampling" were failing, but there are a number of reasons for that and
> they may fail in seemingly unexpected ways.
> 
> My perf binary is built in Thumb2 because that is what the toolchain
> produces by default. Going through the rabbit hole, I found the
> following failure scenarios.
> 
> 1) If __test_function()'s addresss has the Thumb bit set, then we set a
> breakpoint length (bp_len = sizeof(long)) which makes us fail to
> validate the event in hw_breakpoint_arch_parse() and we return -EINVAL
> from SYS_perf_event_open(). This is because the offset computed has a
> value of 1 (function address is e.g:  0x0004c169), but we requested a
> bp_len of 4. The test fails right away.
> 
> 2) If we correct the test such that if addr & 1 == true then we set
> bp_len = 2, then we can see that the test runs to completion, but the
> perf breakpoint event count returns 0 and indeed, no SIGIO is ever
> delivered. This is presumably because of the alignment_mask value of 0x3
> in hw_breakpoint_arch_parse() which would strip the Thumb bit and not
> allow matching it when set assign info->address &= ~alignment_mask. We
> would indeed not have the HW hit that breakpoint at all.
> 
> 3) If we keep the fix from 2) and also change the the alignment_mask to
> 0x2 to preserve the Thumb bit, then we can run into what is described as
> 4) below.
> 
> 4) if __test_function()'s address does not have the Thumb bit set (which
> surprisingly can happen even if test_function does, go figure), then we
> will set a bp_len = 4, and then we are just stuck in an infinite SIGIO
> delivery that looks like this:
> 
> [pid  1859] perf_event_open(0xbebee790, 0, -1, -1, 0x8 /* PERF_FLAG_???
> */) = 3
> [pid  1859] fcntl64(3, F_SETFL, O_RDWR|O_NONBLOCK|O_ASYNC) = 0
> [pid  1859] fcntl64(3, F_SETSIG, 0x1d)  = 0
> [pid  1859] fcntl64(3, F_SETOWN, 1859)  = 0
> [pid  1859] ioctl(3, PERF_EVENT_IOC_RESET, 0) = 0
> [pid  1859] ioctl(3, PERF_EVENT_IOC_ENABLE, 0) = 0
> [pid  1859] --- SIGIO {si_signo=SIGIO, si_code=POLL_IN, si_band=65} ---
> [pid  1859] rt_sigreturn()
> 
> and on and on, we can't even see gettimeofday() begin called in that case.
> 
> This is observable on both 4.9.135 and 4.19 on ARMv7 and ARMv8 CPUs
> running in AArch32.
> 
> I am not clear how to fix that properly, since there appears to be a
> nesting of problems here.

This came up a few years ago iirc and I think most of this boils down to the
fact that we require the overflow handler to do the stepping on arm/arm64,
which is relied upon by GDB/ptrace. The hw_breakpoint code is a complete
disaster so my preference would be to rip out the perf part and just
implement something directly in ptrace, but it's a pretty horrible job.

Are you actually using the perf interface to hw_breakpoint for something
useful?

Will