diff mbox series

[RFC,v2,2/2] selftests/x86: sysret_rip: Add more syscall tests with respect to `%rcx` and `%r11`

Message ID 20230124022729.596997-3-ammarfaizi2@gnuweeb.org (mailing list archive)
State New
Headers show
Series selftests/x86: sysret_rip update for FRED system | expand

Commit Message

Ammar Faizi Jan. 24, 2023, 2:27 a.m. UTC 
From: Ammar Faizi <ammarfaizi2@gnuweeb.org>

Test that:

 - "syscall" in a FRED system doesn't clobber %rcx and %r11.
 - "syscall" in a non-FRED system sets %rcx=%rip and %r11=%rflags.

Test them out with a trivial system call like __NR_getppid and friends
which are extremely likely to return with SYSRET on an IDT system; check
that it returns a nonnegative value and then save the result.

Link: https://lore.kernel.org/lkml/25b96960-a07e-a952-5c23-786b55054126@zytor.com
Co-developed-by: "H. Peter Anvin" <hpa@zytor.com>
Signed-off-by: Ammar Faizi <ammarfaizi2@gnuweeb.org>
---

Need hpa's sign off.

 tools/testing/selftests/x86/sysret_rip.c | 9 +++++++++
 1 file changed, 9 insertions(+)

Comments

H. Peter Anvin Jan. 24, 2023, 6:16 a.m. UTC | #1 
On 1/23/23 18:27, Ammar Faizi wrote:
> From: Ammar Faizi <ammarfaizi2@gnuweeb.org>
> 
> Test that:
> 
>   - "syscall" in a FRED system doesn't clobber %rcx and %r11.
>   - "syscall" in a non-FRED system sets %rcx=%rip and %r11=%rflags.
> 
> Test them out with a trivial system call like __NR_getppid and friends
> which are extremely likely to return with SYSRET on an IDT system; check
> that it returns a nonnegative value and then save the result.
> 

"Nonnegative" here should be "valid"; it is an implementation detail 
that the error value is -1.

However, you are not checking that you don't get a mix of REGS_SAVED and 
REGS_SYSRET, which is a major part of the point.

	-hpa
Ammar Faizi Jan. 24, 2023, 6:41 a.m. UTC | #2 
On Mon, Jan 23, 2023 at 10:16:01PM -0800, H. Peter Anvin wrote:
> On 1/23/23 18:27, Ammar Faizi wrote:
> > Test that:
> > 
> >   - "syscall" in a FRED system doesn't clobber %rcx and %r11.
> >   - "syscall" in a non-FRED system sets %rcx=%rip and %r11=%rflags.
> > 
> > Test them out with a trivial system call like __NR_getppid and friends
> > which are extremely likely to return with SYSRET on an IDT system; check
> > that it returns a nonnegative value and then save the result.
> > 
> 
> "Nonnegative" here should be "valid"; it is an implementation detail that
> the error value is -1.

Copy-paste error, will fix that!

> However, you are not checking that you don't get a mix of REGS_SAVED and
> REGS_SYSRET, which is a major part of the point.

Good point!

What do you think of adding this on top of patch #1?

diff --git a/tools/testing/selftests/x86/sysret_rip.c b/tools/testing/selftests/x86/sysret_rip.c
index 75c72d34dbc5840c..3da827713831acbc 100644
--- a/tools/testing/selftests/x86/sysret_rip.c
+++ b/tools/testing/selftests/x86/sysret_rip.c
@@ -47,11 +47,14 @@ static const unsigned long rcx_sentinel = 0x5ca1ab1e0b57ac1e;
 static const unsigned long rflags_sentinel = 0x200a93;
 
 enum regs_ok {
-	REGS_ERROR  = -1,	/* Invalid register contents */
-	REGS_SAVED  =  0,	/* Registers properly preserved */
-	REGS_SYSRET =  1	/* Registers match syscall/sysret */
+	REGS_INIT_VAL	= -2,	/* For init value checker, never returned */
+	REGS_ERROR 	= -1,	/* Invalid register contents */
+	REGS_SAVED 	=  0,	/* Registers properly preserved */
+	REGS_SYSRET	=  1	/* Registers match syscall/sysret */
 };
 
+static enum regs_ok regs_ok_state = REGS_INIT_VAL;
+
 /*
  * Returns:
  *  0 = %rcx and %r11 preserved.
@@ -86,6 +89,7 @@ static long do_syscall(long nr_syscall, unsigned long arg1, unsigned long arg2,
 	register unsigned long r9 asm("%r9");
 	register void *rsp asm("%rsp");
 	unsigned long rcx, rbx;
+	enum regs_ok ret;
 
 	r11 = r11_sentinel;
 	rcx = rcx_sentinel;
@@ -124,7 +128,14 @@ static long do_syscall(long nr_syscall, unsigned long arg1, unsigned long arg2,
 	 * - "syscall" in a non-FRED system sets %rcx=%rip and %r11=%rflags.
 	 *
 	 */
-	assert(check_regs_result(r11, rcx, rbx) != REGS_ERROR);
+	ret = check_regs_result(r11, rcx, rbx);
+	assert(ret != REGS_ERROR);
+
+	if (regs_ok_state == REGS_INIT_VAL)
+		regs_ok_state = ret;
+	else
+		assert(ret == regs_ok_state);
+
 	return nr_syscall;
 }
Ammar Faizi Jan. 24, 2023, 6:47 a.m. UTC | #3 
On Tue, Jan 24, 2023 at 01:41:30PM +0700, Ammar Faizi wrote:
> On Mon, Jan 23, 2023 at 10:16:01PM -0800, H. Peter Anvin wrote:
> > However, you are not checking that you don't get a mix of REGS_SAVED and
> > REGS_SYSRET, which is a major part of the point.
> 
> Good point!
> 
> What do you think of adding this on top of patch #1?
> 
> diff --git a/tools/testing/selftests/x86/sysret_rip.c b/tools/testing/selftests/x86/sysret_rip.c
> index 75c72d34dbc5840c..3da827713831acbc 100644
> --- a/tools/testing/selftests/x86/sysret_rip.c
> +++ b/tools/testing/selftests/x86/sysret_rip.c
> @@ -47,11 +47,14 @@ static const unsigned long rcx_sentinel = 0x5ca1ab1e0b57ac1e;
>  static const unsigned long rflags_sentinel = 0x200a93;
>  
>  enum regs_ok {
> -	REGS_ERROR  = -1,	/* Invalid register contents */
> -	REGS_SAVED  =  0,	/* Registers properly preserved */
> -	REGS_SYSRET =  1	/* Registers match syscall/sysret */
> +	REGS_INIT_VAL	= -2,	/* For init value checker, never returned */
> +	REGS_ERROR 	= -1,	/* Invalid register contents */
> +	REGS_SAVED 	=  0,	/* Registers properly preserved */
> +	REGS_SYSRET	=  1	/* Registers match syscall/sysret */
>  };
>  
> +static enum regs_ok regs_ok_state = REGS_INIT_VAL;
> +
>  /*
>   * Returns:
>   *  0 = %rcx and %r11 preserved.
> @@ -86,6 +89,7 @@ static long do_syscall(long nr_syscall, unsigned long arg1, unsigned long arg2,
>  	register unsigned long r9 asm("%r9");
>  	register void *rsp asm("%rsp");
>  	unsigned long rcx, rbx;
> +	enum regs_ok ret;
>  
>  	r11 = r11_sentinel;
>  	rcx = rcx_sentinel;
> @@ -124,7 +128,14 @@ static long do_syscall(long nr_syscall, unsigned long arg1, unsigned long arg2,
>  	 * - "syscall" in a non-FRED system sets %rcx=%rip and %r11=%rflags.
>  	 *
>  	 */
> -	assert(check_regs_result(r11, rcx, rbx) != REGS_ERROR);
> +	ret = check_regs_result(r11, rcx, rbx);
> +	assert(ret != REGS_ERROR);
> +
> +	if (regs_ok_state == REGS_INIT_VAL)
> +		regs_ok_state = ret;
> +	else
> +		assert(ret == regs_ok_state);
> +
>  	return nr_syscall;
>  }
>  

And oh, on top of that. Add a comment, just to make it clear...

diff --git a/tools/testing/selftests/x86/sysret_rip.c b/tools/testing/selftests/x86/sysret_rip.c
index 3da827713831acbc..3d783f5baee1b66a 100644
--- a/tools/testing/selftests/x86/sysret_rip.c
+++ b/tools/testing/selftests/x86/sysret_rip.c
@@ -131,6 +131,10 @@ static long do_syscall(long nr_syscall, unsigned long arg1, unsigned long arg2,
        ret = check_regs_result(r11, rcx, rbx);
        assert(ret != REGS_ERROR);
 
+       /*
+        * Check that we don't get a mix of REGS_SAVED and REGS_SYSRET.
+        * Need at least 2 times 'syscall' invoked from this function.
+        */
        if (regs_ok_state == REGS_INIT_VAL)
                regs_ok_state = ret;
        else
H. Peter Anvin Jan. 24, 2023, 9:07 a.m. UTC | #4 
On January 23, 2023 10:41:20 PM PST, Ammar Faizi <ammarfaizi2@gnuweeb.org> wrote:
>On Mon, Jan 23, 2023 at 10:16:01PM -0800, H. Peter Anvin wrote:
>> On 1/23/23 18:27, Ammar Faizi wrote:
>> > Test that:
>> > 
>> >   - "syscall" in a FRED system doesn't clobber %rcx and %r11.
>> >   - "syscall" in a non-FRED system sets %rcx=%rip and %r11=%rflags.
>> > 
>> > Test them out with a trivial system call like __NR_getppid and friends
>> > which are extremely likely to return with SYSRET on an IDT system; check
>> > that it returns a nonnegative value and then save the result.
>> > 
>> 
>> "Nonnegative" here should be "valid"; it is an implementation detail that
>> the error value is -1.
>
>Copy-paste error, will fix that!
>
>> However, you are not checking that you don't get a mix of REGS_SAVED and
>> REGS_SYSRET, which is a major part of the point.
>
>Good point!
>
>What do you think of adding this on top of patch #1?
>
>diff --git a/tools/testing/selftests/x86/sysret_rip.c b/tools/testing/selftests/x86/sysret_rip.c
>index 75c72d34dbc5840c..3da827713831acbc 100644
>--- a/tools/testing/selftests/x86/sysret_rip.c
>+++ b/tools/testing/selftests/x86/sysret_rip.c
>@@ -47,11 +47,14 @@ static const unsigned long rcx_sentinel = 0x5ca1ab1e0b57ac1e;
> static const unsigned long rflags_sentinel = 0x200a93;
> 
> enum regs_ok {
>-	REGS_ERROR  = -1,	/* Invalid register contents */
>-	REGS_SAVED  =  0,	/* Registers properly preserved */
>-	REGS_SYSRET =  1	/* Registers match syscall/sysret */
>+	REGS_INIT_VAL	= -2,	/* For init value checker, never returned */
>+	REGS_ERROR 	= -1,	/* Invalid register contents */
>+	REGS_SAVED 	=  0,	/* Registers properly preserved */
>+	REGS_SYSRET	=  1	/* Registers match syscall/sysret */
> };
> 
>+static enum regs_ok regs_ok_state = REGS_INIT_VAL;
>+
> /*
>  * Returns:
>  *  0 = %rcx and %r11 preserved.
>@@ -86,6 +89,7 @@ static long do_syscall(long nr_syscall, unsigned long arg1, unsigned long arg2,
> 	register unsigned long r9 asm("%r9");
> 	register void *rsp asm("%rsp");
> 	unsigned long rcx, rbx;
>+	enum regs_ok ret;
> 
> 	r11 = r11_sentinel;
> 	rcx = rcx_sentinel;
>@@ -124,7 +128,14 @@ static long do_syscall(long nr_syscall, unsigned long arg1, unsigned long arg2,
> 	 * - "syscall" in a non-FRED system sets %rcx=%rip and %r11=%rflags.
> 	 *
> 	 */
>-	assert(check_regs_result(r11, rcx, rbx) != REGS_ERROR);
>+	ret = check_regs_result(r11, rcx, rbx);
>+	assert(ret != REGS_ERROR);
>+
>+	if (regs_ok_state == REGS_INIT_VAL)
>+		regs_ok_state = ret;
>+	else
>+		assert(ret == regs_ok_state);
>+
> 	return nr_syscall;
> }
> 

Works for me, although I would call it REGS_UNDEFINED myself.
Ammar Faizi Jan. 24, 2023, 9:12 a.m. UTC | #5 
On Tue, Jan 24, 2023 at 01:07:38AM -0800, H. Peter Anvin wrote:
> Works for me, although I would call it REGS_UNDEFINED myself.

I'll call it REGS_UNDEFINED too, in v3.
diff mbox series

Patch

diff --git a/tools/testing/selftests/x86/sysret_rip.c b/tools/testing/selftests/x86/sysret_rip.c
index 550bc4e69ac46997..75c72d34dbc5840c 100644
--- a/tools/testing/selftests/x86/sysret_rip.c
+++ b/tools/testing/selftests/x86/sysret_rip.c
@@ -252,8 +252,17 @@  static void test_syscall_fallthrough_to(unsigned long ip)
 	printf("[OK]\tWe survived\n");
 }
 
+static void test_syscall_rcx_r11(void)
+{
+	do_syscall(__NR_getpid, 0, 0, 0, 0, 0, 0);
+	do_syscall(__NR_gettid, 0, 0, 0, 0, 0, 0);
+	do_syscall(__NR_getppid, 0, 0, 0, 0, 0, 0);
+}
+
 int main()
 {
+	test_syscall_rcx_r11();
+
 	/*
 	 * When the kernel returns from a slow-path syscall, it will
 	 * detect whether SYSRET is appropriate.  If it incorrectly