Messages in this thread |  | | Date | Thu, 18 Jun 2020 14:03:32 +0100 | From | Mark Rutland <> | Subject | Re: [PATCH][V3] arm64: perf: Get the wrong PC value in REGS_ABI_32 mode |
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On Thu, May 28, 2020 at 08:54:19AM +0100, Will Deacon wrote: > On Thu, May 28, 2020 at 09:06:07AM +0800, Jiping Ma wrote: > > On 05/27/2020 11:19 PM, Mark Rutland wrote: > > > On Wed, May 27, 2020 at 09:33:00AM +0800, Jiping Ma wrote: > > > > On 05/26/2020 06:26 PM, Mark Rutland wrote: > > > > > On Mon, May 11, 2020 at 10:52:07AM +0800, Jiping Ma wrote: > > > > This modification can not fix our issue,� we need > > > > perf_reg_abi(current) == PERF_SAMPLE_REGS_ABI_32 to judge if it is 32-bit > > > > task or not, > > > > then return the correct PC value. > > > I must be missing something here. > > > > > > The core code perf_reg_abi(task) is called with the task being sampled, > > > and the regs are from the task being sampled. For a userspace sample for > > > a compat task, compat_user_mode(regs) should be equivalent to the > > > is_compat_thread(task_thread_info(task)) check. > > > > > > What am I missing? > > This issue caused by PC value is not correct. regs are sampled in function > > perf_output_sample_regs, that call perf_reg_value(regs, bit) to get PC > > value. > > PC value is regs[15] in perf_reg_value() function. it should be regs[32]. > > > > perf_output_sample_regs(struct perf_output_handle *handle, > > ����������������������� struct pt_regs *regs, u64 mask) > > { > > ������� int bit; > > ������� DECLARE_BITMAP(_mask, 64); > > > > ������� bitmap_from_u64(_mask, mask); > > ������� for_each_set_bit(bit, _mask, sizeof(mask) * BITS_PER_BYTE) { > > ��������������� u64 val; > > > > ��������������� val = perf_reg_value(regs, bit); > > ��������������� perf_output_put(handle, val); > > ������� } > > } > > Yes, but Mark's point is that checking 'compat_user_mode(regs)' should be > exactly the same as checking 'perf_reg_abi(current) == PERF_SAMPLE_REGS_ABI_32'. > Are you saying that's not the case? If so, please can you provide an example > of when they are different? > > Leaving that aside for a second, I also think it's reasonable to question > whether this whole interface is busted or not. I looked at it last night but > struggled to work out what it's supposed to do. Consider these three > scenarios, all under an arm64 kernel: > > 1. 64-bit perf + 64-bit application being profiled > 2. 64-bit perf + 32-bit application being profiled > 3. 32-bit perf + 32-bit application being profiled > > It looks like the current code is a bodge to try to handle both (2) and > (3) at the same time: > > - In case (3), userspace only asks about registers 0-15 > - In case (2), we fudge the higher registers so that 64-bit SP and LR > hold the 32-bit values as a bodge to allow a 64-bit dwarf unwinder > to unwind the stack
I think the fudging is nonsensical to begin with, as I would have expected that PERF_REGS_ABI_32 should be the same layout regardless of consumer (and therefore should be identical to the 32-bit arm native format). I realise that doesn't change that we might be stuck with it...
> So the idea behind the patch looks fine because case (3) is expecting the PC > in register 15 and instead gets 0, but the temptation is to clean this up so > that cases (2) and (3) report the same data to userspace (along the lines of > Mark's patch), namely only the first 16 registers with the PC moved down. We > can only do that if the unwinder is happy, which it might be if it only ever > looks up dwarf register numbers based on the unwind tables in the binary. > Somebody would need to dig into that.
Agreed; I will try to figure out what the perf tool does in the three cases above. I would be grateful if others could take a look too.
Another slightly scary thought: what happens for a 32-bit perf with a 64-bit application being profiled? I don't see how that'd be forbidden, but I also don't see how it'd work.
> Otherwise, if it generates unconditional references to things like > register 30 to grab the link register, then we're stuck with the bodge > and need to special-case the PC.
I agree that in that case we'd have to keep the existing bodge, and we'd have to special-case the PC, but I'd prefer to split the logic for case 1 into a separate function for cases 2 and 3 so that we can more easily avoid getting this more confused.
Let's figure out what userspace does first...
Thanks, Mark.
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