From: Roland McGrath 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. Signed-off-by: Roland McGrath Cc: Andi Kleen Signed-off-by: Andrew Morton --- arch/x86_64/ia32/ia32entry.S | 17 ++++------------- 1 files changed, 4 insertions(+), 13 deletions(-) diff -puN arch/x86_64/ia32/ia32entry.S~x86-64-ptrace-ia32-bp-fix arch/x86_64/ia32/ia32entry.S --- 25/arch/x86_64/ia32/ia32entry.S~x86-64-ptrace-ia32-bp-fix Wed Jul 6 13:23:35 2005 +++ 25-akpm/arch/x86_64/ia32/ia32entry.S Wed Jul 6 13:23:35 2005 @@ -102,6 +102,7 @@ sysenter_do_call: .byte 0xf, 0x35 sysenter_tracesys: + movl %r9d,%ebp SAVE_REST CLEAR_RREGS movq $-ENOSYS,RAX(%rsp) /* really needed? */ @@ -109,13 +110,7 @@ sysenter_tracesys: call syscall_trace_enter LOAD_ARGS ARGOFFSET /* reload args from stack in case ptrace changed it */ RESTORE_REST - movl %ebp, %ebp - /* no need to do an access_ok check here because rbp has been - 32bit zero extended */ -1: movl (%rbp),%r9d - .section __ex_table,"a" - .quad 1b,ia32_badarg - .previous + movl %ebp,%r9d jmp sysenter_do_call CFI_ENDPROC @@ -183,6 +178,7 @@ cstar_do_call: sysretl cstar_tracesys: + movl %r9d,%ebp SAVE_REST CLEAR_RREGS movq $-ENOSYS,RAX(%rsp) /* really needed? */ @@ -191,12 +187,7 @@ cstar_tracesys: LOAD_ARGS ARGOFFSET /* reload args from stack in case ptrace changed it */ RESTORE_REST movl RSP-ARGOFFSET(%rsp), %r8d - /* no need to do an access_ok check here because r8 has been - 32bit zero extended */ -1: movl (%r8),%r9d - .section __ex_table,"a" - .quad 1b,ia32_badarg - .previous + movl %ebp,%r9d jmp cstar_do_call ia32_badarg: _