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

[Bae, Chang Seok]

On Wed, 2020-10-07 at 11:05 +0100, Dave Martin wrote:
> On Tue, Oct 06, 2020 at 05:45:24PM +0000, Bae, Chang Seok wrote:
> > On Mon, 2020-10-05 at 14:42 +0100, Dave Martin wrote:
> > > On Tue, Sep 29, 2020 at 01:57:43PM -0700, Chang S. Bae wrote:
> > > > 
> > > > +/*
> > > > + * 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.
> > > 
> > 
> > Yeah, I saw that. The ARM code adds the max padding, too:
> > 
> > 	signal_minsigstksz = sigframe_size(&user) +
> > 		round_up(sizeof(struct frame_record), 16) +
> > 		16; /* max alignment padding */
> > 
> > 
> > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/arm64/kernel/signal.c#n973
> > 
> > > 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.
> > > 
> > 
> > Because the last state size of XSAVE may not be 64B aligned, the (reported)
> > sum of xstate size here does not guarantee 64B alignment.
> > 
> > > I'm can't think of a clear argument as to why it really matters, though.
> > 
> > We care about the start of XSAVE buffer for the XSAVE instructions, to be
> > 64B-aligned.
> 
> Ah, I see.  That makes sense.
> 
> For arm64, there is no additional alignment padding inside the frame,
> only the padding inserted after the frame to ensure that the base
> address is 16-byte aligned.
> 
> However, I wonder whether people will tend to assume that AT_MINSIGSTKSZ
> is a sensible (if minimal) amount of stack to allocate.  Allocating an
> odd number of bytes, or any amount that isn't a multiple of the
> architecture's preferred (or mandated) stack alignment probably doesn't
> make sense.
> 
> AArch64 has a mandatory stack alignment of 16 bytes; I'm not sure about
> x86.

The x86 ABI looks to require 16-byte alignment (for both 32-/64-bit modes).
FWIW, the 32-bit ABI got changed from 4-byte alignement.

Thank you for brining up the point. Ack. The kernel is expected to return a
16-byte aligned size. I made this change after a discussion with H.J.:

diff --git a/arch/x86/kernel/signal.c b/arch/x86/kernel/signal.c
index c042236ef52e..52815d7c08fb 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;
 }
@@ -404,7 +409,7 @@ static int __setup_rt_frame(int sig, struct ksignal
*ksig,
 	unsafe_put_sigcontext(&frame->uc.uc_mcontext, fp, regs, set,
Efault);
 	unsafe_put_sigmask(set, frame, Efault);
 	user_access_end();
-	
+
 	if (copy_siginfo_to_user(&frame->info, &ksig->info))
 		return -EFAULT;
 
@@ -685,6 +690,8 @@ SYSCALL_DEFINE0(rt_sigreturn)
  * -------------------------
  * | fsave header          |
  * -------------------------
+ * | alignment padding     |
+ * -------------------------
  * | siginfo + ucontext    |
  * -------------------------
  */
@@ -710,7 +717,12 @@ void __init init_sigframe_size(void)
 	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);
 }
 
 unsigned long get_sigframe_size(void)


Thanks,
Chang