Re: [PATCH] arm64: stacktrace: don't trace arch_stack_walk()

[Catalin Marinas <catalin.marinas@arm.com>]

On Fri, Mar 19, 2021 at 06:41:06PM +0000, Mark Rutland wrote:
> We recently converted arm64 to use arch_stack_walk() in commit:
> 
>   5fc57df2f6fd ("arm64: stacktrace: Convert to ARCH_STACKWALK")
> 
> The core stacktrace code expects that (when tracing the current task)
> arch_stack_walk() starts a trace at its caller, and does not include
> itself in the trace. However, arm64's arch_stack_walk() includes itself,
> and so traces include one more entry than callers expect. The core
> stacktrace code which calls arch_stack_walk() tries to skip a number of
> entries to prevent itself appearing in a trace, and the additional entry
> prevents skipping one of the core stacktrace functions, leaving this in
> the trace unexpectedly.
> 
> We can fix this by having arm64's arch_stack_walk() begin the trace with
> its caller. The first value returned by the trace will be
> __builtin_return_address(0), i.e. the caller of arch_stack_walk(). The
> first frame record to be unwound will be __builtin_frame_address(1),
> i.e. the caller's frame record. To prevent surprises, arch_stack_walk()
> is also marked noinline.
> 
> While __builtin_frame_address(1) is not safe in portable code, local GCC
> developers have confirmed that it is safe on arm64. To find the caller's
> frame record, the builtin can safely dereference the current function's
> frame record or (in theory) could stash the original FP into another GPR
> at function entry time, neither of which are problematic.
> 
> Prior to this patch, the tracing code would unexpectedly show up in
> traces of the current task, e.g.
> 
> | # cat /proc/self/stack
> | [<0>] stack_trace_save_tsk+0x98/0x100
> | [<0>] proc_pid_stack+0xb4/0x130
> | [<0>] proc_single_show+0x60/0x110
> | [<0>] seq_read_iter+0x230/0x4d0
> | [<0>] seq_read+0xdc/0x130
> | [<0>] vfs_read+0xac/0x1e0
> | [<0>] ksys_read+0x6c/0xfc
> | [<0>] __arm64_sys_read+0x20/0x30
> | [<0>] el0_svc_common.constprop.0+0x60/0x120
> | [<0>] do_el0_svc+0x24/0x90
> | [<0>] el0_svc+0x2c/0x54
> | [<0>] el0_sync_handler+0x1a4/0x1b0
> | [<0>] el0_sync+0x170/0x180
> 
> After this patch, the tracing code will not show up in such traces:
> 
> | # cat /proc/self/stack
> | [<0>] proc_pid_stack+0xb4/0x130
> | [<0>] proc_single_show+0x60/0x110
> | [<0>] seq_read_iter+0x230/0x4d0
> | [<0>] seq_read+0xdc/0x130
> | [<0>] vfs_read+0xac/0x1e0
> | [<0>] ksys_read+0x6c/0xfc
> | [<0>] __arm64_sys_read+0x20/0x30
> | [<0>] el0_svc_common.constprop.0+0x60/0x120
> | [<0>] do_el0_svc+0x24/0x90
> | [<0>] el0_svc+0x2c/0x54
> | [<0>] el0_sync_handler+0x1a4/0x1b0
> | [<0>] el0_sync+0x170/0x180
> 
> Erring on the side of caution, I've given this a spin with a bunch of
> toolchains, verifying the output of /proc/self/stack and checking that
> the assembly looked sound. For GCC (where we require version 5.1.0 or
> later) I tested with the kernel.org crosstool binares for versions
> 5.5.0, 6.4.0, 6.5.0, 7.3.0, 7.5.0, 8.1.0, 8.3.0, 8.4.0, 9.2.0, and
> 10.1.0. For clang (where we require version 10.0.1 or later) I tested
> with the llvm.org binary releases of 11.0.0, and 11.0.1.
> 
> Fixes: 5fc57df2f6fd ("arm64: stacktrace: Convert to ARCH_STACKWALK")
> Signed-off-by: Mark Rutland <mark.rutland@arm.com>
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Chen Jun <chenjun102@huawei.com>
> Cc: Marco Elver <elver@google.com>
> Cc: Mark Brown <broonie@kernel.org>
> Cc: Will Deacon <will@kernel.org>

Thanks Mark. I think we should add a cc stable, just with Fixes doesn't
always seem to end up in a stable kernel:

Cc: <stable@vger.kernel.org> # 5.10.x

With that:

Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>

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