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