[bug 20412] PTRACE_SINGLESTEP'ing an instruction that calls user helpers never stops the child on ARM

[Nicolas Pitre]

On Sun, 17 Oct 2010, Timo Juhani Lindfors wrote:

> Hi,
> 
> [ Also available as https://bugzilla.kernel.org/show_bug.cgi?id=20412 ]
> 
> On ARM glibc contains
> 
> 0x4009b864 <getchar+240>:    sub     pc, r3, #63     ; 0x3f
> 
> that jumps to the kernel provided user helper page to access __kernel_cmpxchg
> at 0xffff0fc0. If I try to PTRACE_SINGLESTEP this instruction the child never
> stops. Parent keeps wait()ing forever but child effectively escapes the tracing
> completely and continues executing instructions without notifying the parent.
> 
> On ARM single stepping is done by adding a breakpoint to possible branch
> targets of each instruction. I guess when ptrace_set_bpt() tries to
> add_breakpoint() to this page it just fails? (There's even a comment that says
> it can fail and that return value is not checked...)
> 
> Since all user helpers return to LR I think we could teach get_branch_address
> to treat jumps to user helpers as instructions that branch to lr? The attached
> patch works for me but I am not sure if it is a good solution.

It is certainly better than nothing.  

Acked-by: Nicolas Pitre <nicolas.pitre at linaro.org>


Nicolas