Re: [PATCH] Stacktrace in ARM32 architecture has jumped the first 2 layers, which may ignore the display of save_stack_trace_tsk.

[Peter Zijlstra <peterz@infradead.org>]

On Thu, May 30, 2019 at 05:22:19PM +0100, Russell King - ARM Linux admin wrote:
> On Thu, May 30, 2019 at 11:06:39PM +0800, l00383200 wrote:
> > Without optimization, both save_stack_trace_tsk and __save_stack_trace
> > will have stacktrace information in ARM32.
> > 
> > In this situation, "data.skip += 2" operation will skip the first two layers,
> > which may make the stacktrace strange and different from other architectures.
> > 
> > A simple example is as follows:
> > In ARM32 architecture:
> > [<ffffff80083cb3f8>] proc_pid_stack+0xac/0x12c
> > [<ffffff80083c7c70>] proc_single_show+0x5c/0xa8
> > [<ffffff800838aca8>] seq_read+0x130/0x420
> > [<ffffff8008365c54>] __vfs_read+0x60/0x11c
> > [<ffffff80083665dc>] vfs_read+0x8c/0x140
> > [<ffffff800836717c>] SyS_read+0x6c/0xcc
> > [<ffffff8008202cb8>] __sys_trace_return+0x0/0x4
> > [<ffffffffffffffff>] 0xffffffffffffffff
> > 
> > In some other architectures(ARM64):
> > [<ffffff8008209be0>] save_stack_trace_tsk+0x0/0xf0
> > [<ffffff80083cb3f8>] proc_pid_stack+0xac/0x12c
> > [<ffffff80083c7c70>] proc_single_show+0x5c/0xa8
> > [<ffffff800838aca8>] seq_read+0x130/0x420
> > [<ffffff8008365c54>] __vfs_read+0x60/0x11c
> > [<ffffff80083665dc>] vfs_read+0x8c/0x140
> > [<ffffff800836717c>] SyS_read+0x6c/0xcc
> > [<ffffff8008202cb8>] __sys_trace_return+0x0/0x4
> > [<ffffffffffffffff>] 0xffffffffffffffff
> > 
> > Therefore, we'd better just jump only one layer to ensure accuracy and consistency.
> 
> Why do we want to log the function we called to save the stack trace
> _in_ the stack trace?  What useful purpose does it serve?
> 
> I've always taken the attitude that if we want a stack trace from a
> certain point in the function, then that's the point that the stack
> trace should start.  It's entirely sensible.

Agreed, also the .skip interface sucks and is slated for replacement
(whenever we get around to it).