Messages in this thread | | | Date | Thu, 09 Oct 2014 13:29:33 +0900 | From | AKASHI Takahiro <> | Subject | Re: [PATCH v7 2/6] arm64: ptrace: allow tracer to skip a system call |
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On 10/08/2014 11:23 PM, Will Deacon wrote: > On Thu, Oct 02, 2014 at 10:46:12AM +0100, AKASHI Takahiro wrote: >> If tracer specifies -1 as a syscall number, this traced system call should >> be skipped with a value in x0 used as a return value. >> This patch implements this semantics, but there is one restriction here: >> >> when syscall(-1) is issued by user, tracer cannot skip this system call >> and modify a return value at syscall entry. >> >> In order to ease this flavor, we need to take whatever value x0 has as >> a return value, but this might result in a bogus value being returned, >> especially when tracer doesn't do anything against this syscall. >> So we always return ENOSYS instead, while we still have another chance to >> change a return value at syscall exit. >> >> Please also note: >> * syscall entry tracing and syscall exit tracing (ftrace tracepoint and >> audit) are always executed, if enabled, even when skipping a system call >> (that is, -1). >> In this way, we can avoid a potential bug where audit_syscall_entry() >> might be called without audit_syscall_exit() at the previous system call >> being called, that would cause OOPs in audit_syscall_entry(). >> >> * syscallno may also be set to -1 if a fatal signal (SIGKILL) is detected >> in tracehook_report_syscall_entry(), but since a value set to x0 (ENOSYS) >> is not used in this case, we may neglect the case. >> >> Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org> >> --- >> arch/arm64/include/asm/ptrace.h | 8 ++++++++ >> arch/arm64/kernel/entry.S | 4 ++++ >> arch/arm64/kernel/ptrace.c | 23 ++++++++++++++++++++++- >> 3 files changed, 34 insertions(+), 1 deletion(-) >> >> 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.
>> + >> #ifndef __ASSEMBLY__ >> >> /* sizeof(struct user) for AArch32 */ >> diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S >> index f0b5e51..b53a1c5 100644 >> --- a/arch/arm64/kernel/entry.S >> +++ b/arch/arm64/kernel/entry.S >> @@ -25,6 +25,7 @@ >> #include <asm/asm-offsets.h> >> #include <asm/errno.h> >> #include <asm/esr.h> >> +#include <asm/ptrace.h> >> #include <asm/thread_info.h> >> #include <asm/unistd.h> >> >> @@ -671,6 +672,8 @@ ENDPROC(el0_svc) >> __sys_trace: >> mov x0, sp >> bl syscall_trace_enter >> + cmp w0, #-1 // skip the syscall? >> + b.eq __sys_trace_return_skipped >> adr lr, __sys_trace_return // return address >> uxtw scno, w0 // syscall number (possibly new) >> mov x1, sp // pointer to regs >> @@ -685,6 +688,7 @@ __sys_trace: >> >> __sys_trace_return: >> str x0, [sp] // save returned x0 >> +__sys_trace_return_skipped: >> mov x0, sp >> bl syscall_trace_exit >> b ret_to_user >> 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).
>> + * >> + * 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.)
To avoid this incompatibility, there is no way but to always return -1 in this path because the kernel doesn't know whether a debugger let x0 unchanged on purpose or not. This is also the reason why I wanted to have a dedicated ptrace command to set a return value in skipping a system call.
If we don't care about such erroneous (and exceptional) behaviors, we don't need this 'if' statement.
Did I make it clear?
>> + * NOTE: syscallno may also be set to -1 if fatal signal >> + * is detected in tracehook_report_syscall(ENTRY), >> + * but since a value set to x0 here is not used in this >> + * case, we may neglect the case. >> + */ > > I think can you remove thise NOTE, it's not very informative.
Okey. Also remove descriptions from a commit message.
-Takahiro AKASHI
> Will >
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