Re: [PATCH] x86-64: ptrace ia32 BP fix

[Daniel Jacobowitz <drow@false.org>]

On Tue, Jul 05, 2005 at 02:31:15AM -0700, Roland McGrath wrote:
> 
> When the 32-bit vDSO is used to make a system call, the %ebp register for
> the 6th syscall arg has to be loaded from the user stack (where it's pushed
> by the vDSO user code).  The native i386 kernel always does this before
> stopping for syscall tracing, so %ebp can be seen and modified via ptrace
> to access the 6th syscall argument.  The x86-64 kernel fails to do this,
> presenting the stack address to ptrace instead.  This makes the %rbp value
> seen by 64-bit ptrace of a 32-bit process, and the %ebp value seen by a
> 32-bit caller of ptrace, both differ from the native i386 behavior.
> 
> This patch fixes the problem by putting the word loaded from the user stack
> into %rbp before calling syscall_trace_enter, and reloading the 6th syscall
> argument from there afterwards (so ptrace can change it).  This makes the
> behavior match that of i386 kernels.

Wouldn't this  to botch a debugger which supported both backtracing and
PTRACE_SYSCALL, when stopped in a syscall?  We have unwind information
for the VDSO and it's not going to tell us that the kernel has done
something clever to the value of %ebp.


-- 
Daniel Jacobowitz
CodeSourcery, LLC