Re: [RFC PATCH 1/4] x86/signal: Introduce helpers to get the maximum signal frame size

[Dave Martin]

On Tue, Sep 29, 2020 at 01:57:43PM -0700, 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.
> 
> Signed-off-by: Chang S. Bae <chang.seok.bae@xxxxxxxxx>
> Reviewed-by: Len Brown <len.brown@xxxxxxxxx>
> Cc: x86@xxxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> ---
>  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          | 45 +++++++++++++++++++++++++++++++
>  5 files changed, 93 insertions(+)

[...]

> diff --git a/arch/x86/kernel/signal.c b/arch/x86/kernel/signal.c
> index be0d7d4152ec..239a0b23a4b0 100644
> --- a/arch/x86/kernel/signal.c
> +++ b/arch/x86/kernel/signal.c
> @@ -663,6 +663,51 @@ 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          |
> + * -------------------------
> + * | 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 += fpu__get_fpstate_sigframe_size() + MAX_XSAVE_PADDING;

For arm64, we round the worst-case padding up by one.

I can't remember the full rationale for this, but it at least seemed a
bit weird to report a size that is not a multiple of the alignment.

I'm can't think of a clear argument as to why it really matters, though.

[...]

Cheers
---Dave