diff mbox series

[v2,1/4] x86/signal: Introduce helpers to get the maximum signal frame size

Message ID 20201119190237.626-2-chang.seok.bae@intel.com
State Not Applicable
Headers show
Series x86: Improve Minimum Alternate Stack Size | expand

Commit Message

Chang S. Bae Nov. 19, 2020, 7:02 p.m. UTC
Signal frames do not have a fixed format and can vary in size when a number
of things change: support XSAVE features, 32 vs. 64-bit apps. Add the code
to support a runtime method for userspace to dynamically discover how large
a signal stack needs to be.

Introduce a new variable, max_frame_size, and helper functions for the
calculation to be used in a new user interface. Set max_frame_size to a
system-wide worst-case value, instead of storing multiple app-specific
values.

Locate the body of the helper function -- fpu__get_fpstate_sigframe_size()
in fpu/signal.c for its relevance.

Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
Reviewed-by: Len Brown <len.brown@intel.com>
Acked-by: H.J. Lu <hjl.tools@gmail.com>
Cc: x86@kernel.org
Cc: linux-kernel@vger.kernel.org
---
Change from v1:
* Took stack alignment into account for sigframe size (Dave Martin)
---
 arch/x86/include/asm/fpu/signal.h |  2 +
 arch/x86/include/asm/sigframe.h   | 23 ++++++++++++
 arch/x86/kernel/cpu/common.c      |  3 ++
 arch/x86/kernel/fpu/signal.c      | 20 ++++++++++
 arch/x86/kernel/signal.c          | 61 ++++++++++++++++++++++++++++++-
 5 files changed, 107 insertions(+), 2 deletions(-)

Comments

Borislav Petkov Nov. 25, 2020, 11:17 a.m. UTC | #1
On Thu, Nov 19, 2020 at 11:02:34AM -0800, Chang S. Bae wrote:
> Signal frames do not have a fixed format and can vary in size when a number
> of things change: support XSAVE features, 32 vs. 64-bit apps. Add the code
> to support a runtime method for userspace to dynamically discover how large
> a signal stack needs to be.
> 
> Introduce a new variable, max_frame_size, and helper functions for the
> calculation to be used in a new user interface. Set max_frame_size to a
> system-wide worst-case value, instead of storing multiple app-specific
> values.
> 
> Locate the body of the helper function -- fpu__get_fpstate_sigframe_size()
> in fpu/signal.c for its relevance.

This sentence is strange and not needed.

> diff --git a/arch/x86/include/asm/sigframe.h b/arch/x86/include/asm/sigframe.h
> index 84eab2724875..ac77f3f90bc9 100644
> --- a/arch/x86/include/asm/sigframe.h
> +++ b/arch/x86/include/asm/sigframe.h
> @@ -52,6 +52,15 @@ struct rt_sigframe_ia32 {
>  	char retcode[8];
>  	/* fp state follows here */
>  };
> +
> +#define SIZEOF_sigframe_ia32	sizeof(struct sigframe_ia32)
> +#define SIZEOF_rt_sigframe_ia32	sizeof(struct rt_sigframe_ia32)
> +
> +#else
> +
> +#define SIZEOF_sigframe_ia32	0
> +#define SIZEOF_rt_sigframe_ia32	0
> +
>  #endif /* defined(CONFIG_X86_32) || defined(CONFIG_IA32_EMULATION) */
>  
>  #ifdef CONFIG_X86_64
> @@ -81,8 +90,22 @@ struct rt_sigframe_x32 {
>  	/* fp state follows here */
>  };
>  
> +#define SIZEOF_rt_sigframe_x32	sizeof(struct rt_sigframe_x32)
> +
>  #endif /* CONFIG_X86_X32_ABI */
>  
> +#define SIZEOF_rt_sigframe	sizeof(struct rt_sigframe)
> +
> +#else
> +
> +#define SIZEOF_rt_sigframe	0
> +
>  #endif /* CONFIG_X86_64 */
>  
> +#ifndef SIZEOF_rt_sigframe_x32
> +#define SIZEOF_rt_sigframe_x32	0
> +#endif

Those are defined here to be used in only one place -
init_sigframe_size() - where there already is ifdeffery. Just use the
normal sizeof() operator there instead of adding more gunk here.

> diff --git a/arch/x86/kernel/fpu/signal.c b/arch/x86/kernel/fpu/signal.c
> index a4ec65317a7f..9f009525f551 100644
> --- a/arch/x86/kernel/fpu/signal.c
> +++ b/arch/x86/kernel/fpu/signal.c
> @@ -507,6 +507,26 @@ fpu__alloc_mathframe(unsigned long sp, int ia32_frame,
>  
>  	return sp;
>  }
> +
> +unsigned long fpu__get_fpstate_sigframe_size(void)
> +{
> +	unsigned long xstate_size = xstate_sigframe_size();
> +	unsigned long fsave_header_size = 0;
> +
> +	/*
> +	 * This space is needed on (most) 32-bit kernels, or when a 32-bit
> +	 * app is running on a 64-bit kernel. To keep things simple, just
> +	 * assume the worst case and always include space for 'freg_state',
> +	 * even for 64-bit apps on 64-bit kernels. This wastes a bit of
> +	 * space, but keeps the code simple.
> +	 */
> +	if ((IS_ENABLED(CONFIG_IA32_EMULATION) ||
> +	     IS_ENABLED(CONFIG_X86_32)) && use_fxsr())
> +		fsave_header_size = sizeof(struct fregs_state);
> +
> +	return fsave_header_size + xstate_size;
> +}

I guess this can be simplified to:

unsigned long fpu__get_fpstate_sigframe_size(void)
{
        unsigned long ret = xstate_sigframe_size();

        /*
         * This space is needed on (most) 32-bit kernels, or when a 32-bit
         * app is running on a 64-bit kernel. To keep things simple, just
         * assume the worst case and always include space for 'freg_state',
         * even for 64-bit apps on 64-bit kernels. This wastes a bit of
         * space, but keeps the code simple.
         */
        if ((IS_ENABLED(CONFIG_IA32_EMULATION) ||
             IS_ENABLED(CONFIG_X86_32)) && use_fxsr())
                ret += sizeof(struct fregs_state);

        return ret;
}

Also, this function simply gives you the xstate size, there's no need
for "sigframe" in the name.
Maninder Singh via Libc-alpha Nov. 30, 2020, 8:40 p.m. UTC | #2
> On Nov 25, 2020, at 20:17, Borislav Petkov <bp@alien8.de> wrote:
> 
> On Thu, Nov 19, 2020 at 11:02:34AM -0800, Chang S. Bae wrote:
>> diff --git a/arch/x86/include/asm/sigframe.h b/arch/x86/include/asm/sigframe.h
>> index 84eab2724875..ac77f3f90bc9 100644
>> --- a/arch/x86/include/asm/sigframe.h
>> +++ b/arch/x86/include/asm/sigframe.h
>> @@ -52,6 +52,15 @@ struct rt_sigframe_ia32 {
>> 	char retcode[8];
>> 	/* fp state follows here */
>> };
>> +
>> +#define SIZEOF_sigframe_ia32	sizeof(struct sigframe_ia32)
>> +#define SIZEOF_rt_sigframe_ia32	sizeof(struct rt_sigframe_ia32)
>> +
>> +#else
>> +
>> +#define SIZEOF_sigframe_ia32	0
>> +#define SIZEOF_rt_sigframe_ia32	0
>> +
>> #endif /* defined(CONFIG_X86_32) || defined(CONFIG_IA32_EMULATION) */
>> 
>> #ifdef CONFIG_X86_64
>> @@ -81,8 +90,22 @@ struct rt_sigframe_x32 {
>> 	/* fp state follows here */
>> };
>> 
>> +#define SIZEOF_rt_sigframe_x32	sizeof(struct rt_sigframe_x32)
>> +
>> #endif /* CONFIG_X86_X32_ABI */
>> 
>> +#define SIZEOF_rt_sigframe	sizeof(struct rt_sigframe)
>> +
>> +#else
>> +
>> +#define SIZEOF_rt_sigframe	0
>> +
>> #endif /* CONFIG_X86_64 */
>> 
>> +#ifndef SIZEOF_rt_sigframe_x32
>> +#define SIZEOF_rt_sigframe_x32	0
>> +#endif
> 
> Those are defined here to be used in only one place -
> init_sigframe_size() - where there already is ifdeffery. Just use the
> normal sizeof() operator there instead of adding more gunk here.

[ Just want to clarify your comment. ]

Admittedly, this is an (ugly) workaround to avoid compile errors.

E.g. when code is written like this in the function:

	if (IS_ENABLED(CONFIG_X86_X32_ABI))
		size = max(size, sizeof(struct rt_sigframe_x32));

and compile with CONFIG_X86_X32_ABI=n, got such a message:

	"invalid application of 'sizeof' to incomplete type 'struct
	sigframe_ia32’"

While the coding-style doc [1] seems to mention this:

   "However, this approach still allows the C compiler to see the
    code inside the block, and check it for correctness (syntax, 
    types, symbol references, etc). Thus, you still have to use an 
    #ifdef if the code inside the block references symbols that 
    will not exist if the condition is not met.”

In general, putting #ifdef in a C file is advised to avoid. I wonder
whether it is okay to include #ifdef in the C file in this case.

Thanks,
Chang

[1] https://www.kernel.org/doc/html/v5.9/process/coding-style.html
Borislav Petkov Dec. 1, 2020, 2:27 p.m. UTC | #3
On Mon, Nov 30, 2020 at 08:40:32PM +0000, Bae, Chang Seok wrote:
> In general, putting #ifdef in a C file is advised to avoid.

Well, in this case it is arguably better to have the ifdeffery there,
where it is being used.

> I wonder whether it is okay to include #ifdef in the C file in this
> case.

Yes, it's not like this will be the only place where you have #ifdef in
a .c file:

[ ~/kernel/linux> git grep "#ifdef" *.c | wc -l
29401
diff mbox series

Patch

diff --git a/arch/x86/include/asm/fpu/signal.h b/arch/x86/include/asm/fpu/signal.h
index 7fb516b6893a..5bfbf8f2e5a3 100644
--- a/arch/x86/include/asm/fpu/signal.h
+++ b/arch/x86/include/asm/fpu/signal.h
@@ -29,6 +29,8 @@  unsigned long
 fpu__alloc_mathframe(unsigned long sp, int ia32_frame,
 		     unsigned long *buf_fx, unsigned long *size);
 
+unsigned long fpu__get_fpstate_sigframe_size(void);
+
 extern void fpu__init_prepare_fx_sw_frame(void);
 
 #endif /* _ASM_X86_FPU_SIGNAL_H */
diff --git a/arch/x86/include/asm/sigframe.h b/arch/x86/include/asm/sigframe.h
index 84eab2724875..ac77f3f90bc9 100644
--- a/arch/x86/include/asm/sigframe.h
+++ b/arch/x86/include/asm/sigframe.h
@@ -52,6 +52,15 @@  struct rt_sigframe_ia32 {
 	char retcode[8];
 	/* fp state follows here */
 };
+
+#define SIZEOF_sigframe_ia32	sizeof(struct sigframe_ia32)
+#define SIZEOF_rt_sigframe_ia32	sizeof(struct rt_sigframe_ia32)
+
+#else
+
+#define SIZEOF_sigframe_ia32	0
+#define SIZEOF_rt_sigframe_ia32	0
+
 #endif /* defined(CONFIG_X86_32) || defined(CONFIG_IA32_EMULATION) */
 
 #ifdef CONFIG_X86_64
@@ -81,8 +90,22 @@  struct rt_sigframe_x32 {
 	/* fp state follows here */
 };
 
+#define SIZEOF_rt_sigframe_x32	sizeof(struct rt_sigframe_x32)
+
 #endif /* CONFIG_X86_X32_ABI */
 
+#define SIZEOF_rt_sigframe	sizeof(struct rt_sigframe)
+
+#else
+
+#define SIZEOF_rt_sigframe	0
+
 #endif /* CONFIG_X86_64 */
 
+#ifndef SIZEOF_rt_sigframe_x32
+#define SIZEOF_rt_sigframe_x32	0
+#endif
+
+void __init init_sigframe_size(void);
+
 #endif /* _ASM_X86_SIGFRAME_H */
diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 35ad8480c464..6954932272d5 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -58,6 +58,7 @@ 
 #include <asm/intel-family.h>
 #include <asm/cpu_device_id.h>
 #include <asm/uv/uv.h>
+#include <asm/sigframe.h>
 
 #include "cpu.h"
 
@@ -1331,6 +1332,8 @@  static void __init early_identify_cpu(struct cpuinfo_x86 *c)
 
 	fpu__init_system(c);
 
+	init_sigframe_size();
+
 #ifdef CONFIG_X86_32
 	/*
 	 * Regardless of whether PCID is enumerated, the SDM says
diff --git a/arch/x86/kernel/fpu/signal.c b/arch/x86/kernel/fpu/signal.c
index a4ec65317a7f..9f009525f551 100644
--- a/arch/x86/kernel/fpu/signal.c
+++ b/arch/x86/kernel/fpu/signal.c
@@ -507,6 +507,26 @@  fpu__alloc_mathframe(unsigned long sp, int ia32_frame,
 
 	return sp;
 }
+
+unsigned long fpu__get_fpstate_sigframe_size(void)
+{
+	unsigned long xstate_size = xstate_sigframe_size();
+	unsigned long fsave_header_size = 0;
+
+	/*
+	 * This space is needed on (most) 32-bit kernels, or when a 32-bit
+	 * app is running on a 64-bit kernel. To keep things simple, just
+	 * assume the worst case and always include space for 'freg_state',
+	 * even for 64-bit apps on 64-bit kernels. This wastes a bit of
+	 * space, but keeps the code simple.
+	 */
+	if ((IS_ENABLED(CONFIG_IA32_EMULATION) ||
+	     IS_ENABLED(CONFIG_X86_32)) && use_fxsr())
+		fsave_header_size = sizeof(struct fregs_state);
+
+	return fsave_header_size + xstate_size;
+}
+
 /*
  * Prepare the SW reserved portion of the fxsave memory layout, indicating
  * the presence of the extended state information in the memory layout
diff --git a/arch/x86/kernel/signal.c b/arch/x86/kernel/signal.c
index be0d7d4152ec..bae35ff6e744 100644
--- a/arch/x86/kernel/signal.c
+++ b/arch/x86/kernel/signal.c
@@ -212,6 +212,11 @@  do {									\
  * Set up a signal frame.
  */
 
+/* x86 ABI requires 16-byte alignment */
+#define FRAME_ALIGNMENT	16UL
+
+#define MAX_FRAME_PADDING	(FRAME_ALIGNMENT - 1)
+
 /*
  * Determine which stack to use..
  */
@@ -222,9 +227,9 @@  static unsigned long align_sigframe(unsigned long sp)
 	 * Align the stack pointer according to the i386 ABI,
 	 * i.e. so that on function entry ((sp + 4) & 15) == 0.
 	 */
-	sp = ((sp + 4) & -16ul) - 4;
+	sp = ((sp + 4) & -FRAME_ALIGNMENT) - 4;
 #else /* !CONFIG_X86_32 */
-	sp = round_down(sp, 16) - 8;
+	sp = round_down(sp, FRAME_ALIGNMENT) - 8;
 #endif
 	return sp;
 }
@@ -663,6 +668,58 @@  SYSCALL_DEFINE0(rt_sigreturn)
 	return 0;
 }
 
+/*
+ * The FP state frame contains an XSAVE buffer which must be 64-byte aligned.
+ * If a signal frame starts at an unaligned address, extra space is required.
+ * This is the max alignment padding, conservatively.
+ */
+#define MAX_XSAVE_PADDING	63UL
+
+/*
+ * The frame data is composed of the following areas and laid out as:
+ *
+ * -------------------------
+ * | alignment padding     |
+ * -------------------------
+ * | (f)xsave frame        |
+ * -------------------------
+ * | fsave header          |
+ * -------------------------
+ * | alignment padding     |
+ * -------------------------
+ * | siginfo + ucontext    |
+ * -------------------------
+ */
+
+/* max_frame_size tells userspace the worst case signal stack size. */
+static unsigned long __ro_after_init max_frame_size;
+
+void __init init_sigframe_size(void)
+{
+	/*
+	 * Use the largest of possible structure formats. This might
+	 * slightly oversize the frame for 64-bit apps.
+	 */
+
+	if (IS_ENABLED(CONFIG_X86_32) ||
+	    IS_ENABLED(CONFIG_IA32_EMULATION))
+		max_frame_size = max((unsigned long)SIZEOF_sigframe_ia32,
+				     (unsigned long)SIZEOF_rt_sigframe_ia32);
+
+	if (IS_ENABLED(CONFIG_X86_X32_ABI))
+		max_frame_size = max(max_frame_size, (unsigned long)SIZEOF_rt_sigframe_x32);
+
+	if (IS_ENABLED(CONFIG_X86_64))
+		max_frame_size = max(max_frame_size, (unsigned long)SIZEOF_rt_sigframe);
+
+	max_frame_size += MAX_FRAME_PADDING;
+
+	max_frame_size += fpu__get_fpstate_sigframe_size() + MAX_XSAVE_PADDING;
+
+	/* Userspace expects an aligned size. */
+	max_frame_size = round_up(max_frame_size, FRAME_ALIGNMENT);
+}
+
 static inline int is_ia32_compat_frame(struct ksignal *ksig)
 {
 	return IS_ENABLED(CONFIG_IA32_EMULATION) &&