Messages in this thread | | | Date | Wed, 8 Oct 2014 15:23:29 +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 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).
> + > #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/?
> + * > + * 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?
> + * 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.
Will
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