[v2,1/4] x86/signal: Introduce helpers to get the maximum signal frame size
Commit Message
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
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.
> 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
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
@@ -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 */
@@ -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 */
@@ -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
@@ -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
@@ -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) &&