On Thu, Oct 09, 2014 at 05:29:33AM +0100, AKASHI Takahiro wrote:
On 10/08/2014 11:23 PM, Will Deacon wrote:
On Thu, Oct 02, 2014 at 10:46:12AM +0100, AKASHI Takahiro wrote:
diff --git a/arch/arm64/include/asm/ptrace.h b/arch/arm64/include/asm/ptrace.h index 41ed9e1..736ebc3 100644 --- a/arch/arm64/include/asm/ptrace.h +++ b/arch/arm64/include/asm/ptrace.h @@ -65,6 +65,14 @@ #define COMPAT_PT_TEXT_ADDR 0x10000 #define COMPAT_PT_DATA_ADDR 0x10004 #define COMPAT_PT_TEXT_END_ADDR 0x10008
+/*
- System call will be skipped if a syscall number is changed to -1
- with ptrace(PTRACE_SET_SYSCALL).
- Upper 32-bit should be ignored for safe check.
- */
+#define IS_SKIP_SYSCALL(no) ((int)(no & 0xffffffff) == -1)
I don't think this macro is very useful, especially considering that we already use ~0UL explicitly in other places. Just move the comment into syscall_trace_enter and be done with it. I also don't think you need the mask (the cast is enough).
I remember it was necessary for compat PTRACE_SET_SYSCALL, but will double-check it anyway.
Ok, thanks.
diff --git a/arch/arm64/kernel/ptrace.c b/arch/arm64/kernel/ptrace.c index 2842f9f..6b11c6a 100644 --- a/arch/arm64/kernel/ptrace.c +++ b/arch/arm64/kernel/ptrace.c @@ -1126,6 +1126,8 @@ static void tracehook_report_syscall(struct pt_regs *regs,
asmlinkage int syscall_trace_enter(struct pt_regs *regs) {
- unsigned int orig_syscallno = regs->syscallno;
- if (test_thread_flag(TIF_SYSCALL_TRACE)) tracehook_report_syscall(regs, PTRACE_SYSCALL_ENTER);
@@ -1133,7 +1135,26 @@ asmlinkage int syscall_trace_enter(struct pt_regs *regs) trace_sys_enter(regs, regs->syscallno);
audit_syscall_entry(syscall_get_arch(), regs->syscallno,
regs->orig_x0, regs->regs[1], regs->regs[2], regs->regs[3]);
regs->orig_x0, regs->regs[1],regs->regs[2], regs->regs[3]);- if (IS_SKIP_SYSCALL(regs->syscallno) &&
IS_SKIP_SYSCALL(orig_syscallno)) {/** For compatibility, we handles user-issued syscall(-1).Compatibility with what? arch/arm/?
with the case where a process is *not* traced (including audit).
Ok, please make that explicit in the comment.
** RESTRICTION: we can't modify a return value here in this* specific case. In order to ease this flavor, we have to* take whatever value x0 has as a return value, but this* might result in a bogus value being returned.This comment isn't helping me. Are we returning a bogus value or not? If so, why is that acceptable?
I mean that syscall(-1) always returns -1 with ENOSYS.
Let's think about the case that we didn't have this 'if' statement. If a debugger catches an user-issued syscall(-1), but let it go without doing anything (especially changing a value in x0), this syscall will return an original value in x0, which is the first argument of syscall(-1). I mentioned this as "bogus." In this way, a traced process would see a different behavior of syscall(-1). (On arm, this doesn't happen because syscall(-1) is supposed to raise SIGILL.) (On x86, this doesn't happen, probably, because syscall arguments are passed via a stack and we can set a default return value in a register to ENOSYS.)
In which case, it's worth mentioning this in the comment and being explicit that this only applies to -1, as that's the same value we use to indicate that the syscall should be skipped. syscall(-2), for example, doesn't have an issue and will always return -ENOSYS.
Will