Re: kern/60539 (XSAVE changes break ucontext userspace API)

> Date: Thu, 06 Aug 2026 18:10:09 +0700
> From: Robert Elz <[email protected]>
> 
>     Date:        Thu, 6 Aug 2026 02:47:23 +0000
>     From:        Taylor R Campbell <[email protected]>
>     Message-ID:  <[email protected]>
> 
>   | I think such applications are likely to be inherently problematic.  If
>   | you're doing value substitution in an FPU trap handler, and some
>   | AVX512 instruction operating on a 512-bit ZMM vector register traps,
> 
> And if you were doing software emulation of the AVX512 instructions,
> wouldn't the trap handler need to be able to access the (emulated)
> registers ?

Yes, in that case you would absolutely need to know about the new
XSAVE pointer in mcontext_t:

static void
handle_sigfpe(int signo, siginfo_t *si, void *ctx)
{
	ucontext_t *uc = ctx;
	mcontext_t *mc = &uc->uc_mcontext;
	struct xsave_header *xsh =
	    (struct xsave_header *)mc->__fpregs.__xsaveptr;
	size_t xslen = (size_t)mc->__fpregs.__xsavelen;

	... if (xs->xsh_xstate_bv & XCR0_YMM_Hi128) ...
}

The hypothetical I was considering is a handler previously written for
x87/SSE that, say, replaces all double-precision subnormal inputs and
results by zero:

static void
handle_sigfpe(int signo, siginfo_t *si, void *ctx)
{
	ucontext_t *uc = ctx;
	mcontext_t *mc = &uc->uc_mcontext;
	struct fxsave *fx = (struct fxsave *)&mc->__fpregs.__fxsave;

	for (i = 0; i < 16; i++) {
		if (issubnormal(fx->fx_xmm[i].dbl[0]))
			fx->fx_xmm[i].dbl[0] = 0;
		if (issubnormal(fx->fx_xmm[i].dbl[1]))
			fx->fx_xmm[i].dbl[1] = 0;
	}
	/* ditto x87 registers */
}

_Reading_ the FXSAVE state will continue to work, but in a process
that uses AVX instructions -- at least, if the signal is delivered any
time between the first AVX instruction and VZEROUPPER -- _writing_
back changes to the FXSAVE state will be dropped.

But also -- aside from the obvious problems this signal handler has
like not catching any results that have already been written to memory
and not making sense when the registers are used for anything other
than a pair of doubles quantities -- this signal handler is unaware of
the upper parts of the AVX/AVX512 registers ymmN/zmmN whose lower 128
bits alias xmmN, so it will only convert the low 128 bits of each one.

So I'm struggling to imagine a plausible scenario where the change
disabling writeback of the ucontext_t FXSAVE area in the presence of
AVX instructions will actually break something that isn't already
profoundly broken.

In other words, I think it wasn't necessary of me to expand the scope
of the reported PR from `x86 mcontext.h API broke requiring you to
insert a single & in existing programs' (so if we pull up the XSAVE
changes to 9 and 10, the mcontext.h part should perhaps be under
#ifdef _KERNEL) to `ABI change could have broken existing compiled
programs' because the only ones it could have broken must have already
been broken.