Date: Wed, 10 Jul 2019 15:18:52 -0700 From: Mark Millard <marklmi@yahoo.com> To: adr <adr@SDF.ORG> Cc: Robert Crowston via freebsd-arm <freebsd-arm@freebsd.org> Subject: Re: FreeBSD arm EABI5 documentation? Message-ID: <AFF6488F-B2F5-486E-8F14-FAF852DB2C3C@yahoo.com> In-Reply-To: <alpine.NEB.2.21.1907102043040.9461@sdf.lonestar.org> References: <alpine.NEB.2.21.1907101508370.22895@sdf.lonestar.org> <f2967859f68f1d40f260661791126956f48f4d12.camel@freebsd.org> <alpine.NEB.2.21.1907101735020.3201@sdf.lonestar.org> <da6a8f7e596da9bf9015f76798a5575908ad1be4.camel@freebsd.org> <alpine.NEB.2.21.1907101904570.15468@sdf.lonestar.org> <1788e13e706b9fdaf610e4ddd671a5ed715f9dfe.camel@freebsd.org> <alpine.NEB.2.21.1907102043040.9461@sdf.lonestar.org>
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On 2019-Jul-10, at 13:47, adr <adr at SDF.ORG> wrote:
> On Wed, 10 Jul 2019, Ian Lepore wrote:
>
>> That's... odd. The arm spec requires the stack to be 8-byte aligned at
>> any public interface. It's hard to imagine how anything could work
>> properly if it were not, given that the toolchains will assume that 64-
>> bit values are aligned at 64-bit boundaries and will thus generate
>> instructions that require that alignment (require it even if strict
>> alignment checking for most instructions is disabled in the control
>> register). If you could enter a function with the stack only 4-byte
>> aligned, how would the compiler know it's safe to use something like an
>> LDREXD instruction on a local variable allocated on the stack?
>
> I have no idea. The only thing I can assure you is that in the code I'm
> talking about (a forth implementation using SDL2) I've never aligned the
> C stack when passing arguments to external functions. Until now!
Curious, I looked around in NetBSD code and found the likes of:
/* ARM-specific macro to align a stack pointer (downwards). */
#define STACK_ALIGNBYTES (8 - 1)
in netbsd's src/sys/arch/arm/include/param.h .
In src/sys/arch/aarch64/include/param.h there is:
/* AARCH64-specific macro to align a stack pointer (downwards). */
#define STACK_ALIGNBYTES (16 - 1)
So it appears that the kernel does have stack alignment:
src/sys/sys/param.h has . . .
#if defined(_KERNEL) || defined(__EXPOSE_STACK)
. . .
#ifndef STACK_ALIGNBYTES
#define STACK_ALIGNBYTES __ALIGNBYTES
#endif
. . .
#define STACK_ALIGN(sp, bytes) \
((char *)(((unsigned long)(sp)) & ~(bytes)))
. . .
#define STACK_LEN_ALIGN(len, bytes) (((len) + (bytes)) & ~(bytes))
There is code for signal delivery:
void
sendsig_siginfo(const ksiginfo_t *ksi, const sigset_t *mask)
{
. . .
fp = getframe(l, sig, &onstack);
/* make room on the stack */
fp--;
/* make the stack aligned */
fp = (struct sigframe_siginfo *)STACK_ALIGN(fp, STACK_ALIGNBYTES);
. . .
AARCH64 even has for runing 32-bit code:
static void
netbsd32_sendsig_siginfo(const ksiginfo_t *ksi, const sigset_t *mask)
{
. . .
fp = (struct netbsd32_sigframe_siginfo *)sp;
fp = (struct netbsd32_sigframe_siginfo *)STACK_ALIGN(fp - 1, 8);
. . .
(But that last looks wrong: 8 should likely be 8-1, as in
STACK_ALIGNBYTES for 32-bit ARM.)
There is:
static size_t
calcstack(struct execve_data * restrict data, const size_t gaplen)
{
. . .
/* make the stack "safely" aligned */
return STACK_LEN_ALIGN(stacklen, STACK_ALIGNBYTES);
}
I saw other comments mentioning ' stack "safely" aligned' when
looking around.
So it basically matches up with Ian's comments at the kernel vs. user
space interface, despite the probable netbsd32_sendsig_siginfo error.
===
Mark Millard
marklmi at yahoo.com
( dsl-only.net went
away in early 2018-Mar)
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