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Message-ID: <20180526005415.GI4418@port70.net>
Date: Sat, 26 May 2018 02:54:16 +0200
From: Szabolcs Nagy <nsz@...t70.net>
To: musl@...ts.openwall.com
Subject: Re: TLS issue on aarch64
* Phillip Berndt <phillip.berndt@...glemail.com> [2018-05-26 00:20:04 +0200]:
> 2018-05-25 16:50 GMT+02:00 Szabolcs Nagy <nsz@...t70.net>:
> > i think the constraints for tp are:
> >
> > - tp must be aligned to 'tls_align'
> >
> > - tp must be at a small fixed offset from the end
> > of pthread struct (so asm code can access the dtv)
> >
> > - tp + off must be usable memory for tls for off >= 16
> > (this is aarch64 specific)
> >
>
> Hmm.. but these constraints do not explain the extra offset of one
> alignment I'm seeing in the GCC output, do they? If I compile a
tp must be aligned and tp + offset must be aligned too,
but offset >= 16 has to hold.
> program with a single TLS variable with
> __attribute__((aligned(n)) that does nothing but try to reference and
> print said variable, I get the
> following assembler code from GCC:
>
> For n = 0x1000:
>
> 400194: d53bd041 mrs x1, tpidr_el0
> 400198: b0000040 adrp x0, 409000 <__subtf3+0xd18>
> 40019c: 91400421 add x1, x1, #0x1, lsl #12
> 4001a0: 91000021 add x1, x1, #0x0
>
>
> For n = 0x100:
>
> 400194: d53bd041 mrs x1, tpidr_el0
> 400198: b0000040 adrp x0, 409000 <__subtf3+0xd18>
> 40019c: 91400021 add x1, x1, #0x0, lsl #12
> 4001a0: 91040021 add x1, x1, #0x100
>
> For n = 0x10:
>
> 400194: d53bd041 mrs x1, tpidr_el0
> 400198: b0000040 adrp x0, 409000 <__subtf3+0xd18>
> 40019c: 91400021 add x1, x1, #0x0, lsl #12
> 4001a0: 91004021 add x1, x1, #0x10
>
> That's how I came up with the mem += libc.tls_align hack in the first place.
>
indeed you need another alignment there, i came up with the
following fix:
(on mips/ppc i expect it not to change anything: tp is
at a page aligned offset from the end of struct pthread,
so one alignment is enough there, but on aarch64/arm/sh4
this makes a difference, and seems to pass my simple tests)
diff --git a/src/env/__init_tls.c b/src/env/__init_tls.c
index 1c5d98a0..8e70024d 100644
--- a/src/env/__init_tls.c
+++ b/src/env/__init_tls.c
@@ -41,9 +41,12 @@ void *__copy_tls(unsigned char *mem)
#ifdef TLS_ABOVE_TP
dtv = (void **)(mem + libc.tls_size) - (libc.tls_cnt + 1);
- mem += -((uintptr_t)mem + sizeof(struct pthread)) & (libc.tls_align-1);
+ /* Ensure TP is aligned. */
+ mem += -(uintptr_t)TP_ADJ(mem) & (libc.tls_align-1);
td = (pthread_t)mem;
mem += sizeof(struct pthread);
+ /* Ensure TLS is aligned after struct pthread. */
+ mem += -(uintptr_t)mem & (libc.tls_align-1);
for (i=1, p=libc.tls_head; p; i++, p=p->next) {
dtv[i] = mem + p->offset;
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