Re: [RFC PATCH 1/4] x86/signal: Introduce helpers to get the maximum signal frame size
From: Dave Martin <Dave.Martin@arm.com>
Date: 2020-10-05 13:42:38
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linux-arch, lkml
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 <redacted> Reviewed-by: Len Brown <redacted> Cc: x86@kernel.org Cc: linux-kernel@vger.kernel.org --- 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(+)
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quoted hunk ↗ jump to hunk
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