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
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