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Message-ID: <20180502134224.GW1392@brightrain.aerifal.cx>
Date: Wed, 2 May 2018 09:42:24 -0400
From: Rich Felker <dalias@...c.org>
To: musl@...ts.openwall.com
Cc: Patrick Oppenlander <patrick.oppenlander@...il.com>
Subject: Re: Some questions
On Wed, May 02, 2018 at 12:14:41AM +0200, Szabolcs Nagy wrote:
> * Andre McCurdy <armccurdy@...il.com> [2018-05-01 14:49:00 -0700]:
> > On Tue, May 1, 2018 at 10:35 AM, Rich Felker <dalias@...c.org> wrote:
> > > I'm considering applying the attached patch, which would make it so
> > > VLAs don't break thumb syscalls and eliminate the need to force frame
> > > pointer off when building as thumb. This is all a workaround for gcc
> > > being wrong about not letting you use r7, but it seems reasonable and
> > > non-invasive. It just omits r7 from the constraints and uses a temp
> > > register to save/restore it.
> >
> > This seems to fail when compiling src/thread/arm/__set_thread_area.c:
> >
> > {standard input}: Assembler messages:
> > {standard input}:45: Error: invalid constant (f0005) after fixup
> > make: *** [obj/src/thread/arm/__set_thread_area.o] Error 1
> >
> > Without the patch, __set_thread_area() effectively compiles to:
> >
> > __set_thread_area:
> > push {r7, lr}
> > ldr r7, .L2
> > pop {r7, pc}
> > .L2:
> > .word 983045
> >
> > With the patch:
> >
> > __set_thread_area:
> > push {r7, lr}
> > add r7, sp, #0
> > mov r3,r7 ; mov r7,#983045 ; svc 0 ; mov r7,r3
> > pop {r7, pc}
> >
> > ie the immediate value 0xf0005 can't be loaded directly into r7 with a
> > single Thumb2 mov instruction.
> >
> > I tried a quick test to replace the single mov instruction in
> > __asm_syscall() with a movw + movt pair:
> >
>
> i think the syscall can be just inline asm here,
> since __set_thread_area is arm specific code.
>
> in generic code the mov r7,.. hack should work
> and fixes the vla issue.
>
> (alternatively using just "r" operand instead
> of "rI" does not generate immediate, but will
> use more registers/instructions)
The above posted patch used "ri" not "rI". The "I" constraint, which I
switched to after posting the patch, only allows immediates that work
with mov, so the issue does not arise.
Rich
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