Messages in this thread | | | Date | Fri, 10 Oct 2014 12:05:39 +0100 | From | Will Deacon <> | Subject | Re: [PATCH v7 2/6] arm64: ptrace: allow tracer to skip a system call |
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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
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