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Message-ID: <20201012132638.GC32292@arm.com>
Date: Mon, 12 Oct 2020 14:26:39 +0100
From: Dave Martin <Dave.Martin@....com>
To: "Bae, Chang Seok" <chang.seok.bae@...el.com>
Cc: "mingo@...nel.org" <mingo@...nel.org>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
"hjl.tools@...il.com" <hjl.tools@...il.com>,
"bp@...e.de" <bp@...e.de>,
"tglx@...utronix.de" <tglx@...utronix.de>,
"linux-api@...r.kernel.org" <linux-api@...r.kernel.org>,
"libc-alpha@...rceware.org" <libc-alpha@...rceware.org>,
"x86@...nel.org" <x86@...nel.org>,
"luto@...nel.org" <luto@...nel.org>,
"Shankar, Ravi V" <ravi.v.shankar@...el.com>,
"Hansen, Dave" <dave.hansen@...el.com>,
"Luck, Tony" <tony.luck@...el.com>,
"Brown, Len" <len.brown@...el.com>,
"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
"mpe@...erman.id.au" <mpe@...erman.id.au>
Subject: Re: [RFC PATCH 1/4] x86/signal: Introduce helpers to get the maximum
signal frame size
On Thu, Oct 08, 2020 at 10:43:50PM +0000, Bae, Chang Seok wrote:
> 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
> +
You might want () here, to avoid future surpsises.
> /*
> * 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);
> }
[...]
Seems reasonable, I guess.
(I won't comment on the x86 ABI specifics.)
Cheers
---Dave
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