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Message-ID: <20241130032020.GK10433@brightrain.aerifal.cx>
Date: Fri, 29 Nov 2024 22:20:20 -0500
From: Rich Felker <dalias@...c.org>
To: Alex Rønne Petersen <alex@...xrp.com>
Cc: Alexander Monakov <amonakov@...ras.ru>, musl@...ts.openwall.com
Subject: Re: [PATCH] s390x: Mark __tls_get_addr hidden before invoking
it.
On Fri, Nov 29, 2024 at 08:49:00PM +0100, Alex Rønne Petersen wrote:
> On Fri, Nov 29, 2024 at 2:48 PM Rich Felker <dalias@...c.org> wrote:
> >
> > On Sat, Nov 23, 2024 at 01:57:16PM +0100, Alex Rønne Petersen wrote:
> > > On Sat, Nov 23, 2024 at 1:36 PM Alexander Monakov <amonakov@...ras.ru> wrote:
> > > >
> > > > On Sat, 23 Nov 2024, Alex Rønne Petersen wrote:
> > > >
> > > > > On Sat, Nov 23, 2024 at 9:30 AM Alexander Monakov <amonakov@...ras.ru> wrote:
> > > > > >
> > > > > > On Sat, 23 Nov 2024, Alex Rønne Petersen wrote:
> > > > > >
> > > > > > > Similar to what's done for __syscall_ret, __sigsetjmp_tail, etc.. This fixes a
> > > > > > > linker error when building musl libc.so with zig cc.
> > > > > >
> > > > > > Hm, on s390 __tls_get_addr is not used for TLS ABI, so it's fine that it ends up
> > > > > > hidden in libc.so. Unusual.
> > > > > >
> > > > > > (linkers must take the most restrictive visibility from all mentions of a symbol)
> > > > > >
> > > > > > I'm curious, what kind of error with zig cc were you seeing?
> > > > >
> > > > > This:
> > > > >
> > > > > ld.lld: error: relocation R_390_PC32DBL cannot be used against symbol
> > > > > '__tls_get_addr'; recompile with -fPIC
> > > > > >>> defined in obj/src/thread/__tls_get_addr.lo
> > > > > >>> referenced by __tls_get_offset.s:8 (src/thread/s390x/__tls_get_offset.s:8)
> > > > > >>> obj/src/thread/s390x/__tls_get_offset.lo:(.text+0x10)
> > > > >
> > > > > (-fPIC is actually in use.)
> > > > >
> > > > > Presumably this could be fixed in lld, considering GNU ld seems fine
> > > > > with it. But I figured that, since glibc also marks __tls_get_addr
> > > > > hidden for s390x, musl should probably just do the same anyway.
> > > >
> > > > I see, thanks. Your commit message was confusing to me, because unlike
> > > > __syscall_ret and the like, __tls_get_addr is not an internal helper,
> > > > it may not have hidden visibility anywhere except s390. So it felt like
> > > > the commit message was drawing a false parallel.
> > > >
> > > > I would love this to land with a clearer commit message, but that's up
> > > > to Rich and yourself to sort out.
> > >
> > > Yeah, I think that's fair. I wrote the commit message before I
> > > actually investigated in detail how __tls_get_addr is supposed to be
> > > handled for s390x.
> > >
> > > Should I re-send the patch with an updated commit message, or how is
> > > this usually handled?
> >
> > While s390x doesn't need __tls_get_addr to be a public symbol, I'd
> > kinda prefer not to have an arch-specific hack to make it hidden.
> > Looking at the code, it's got to be significantly gratuitously slow
> > having __tls_get_offset making a second function call to
> > __tls_get_addr, setting up a stack frame and all.
> >
> > The __tls_get_offset code dates back to 2016 when it was actually
> > necessary to call into C code in case new TLS needed to be installed.
> > Since 2019 (9d44b6460a) that's not necessary, so I think we could just
> > open code the asm for __tls_get_offset entirely and have it be
> > decently fast.
>
> That sounds reasonable. I don't have a ton of experience with writing
> s390x assembly, though. I can do the obvious thing and extract the
> compiled logic from __tls_get_addr without the calling convention
> fluff. Would that be sufficient?
That's what I was looking at doing. Basically just compiling a
modified version of __tls_get_addr that subtracts the thread pointer,
then prepending the code to load the index address from the GOT
pointer argument in r12.
A further optimization later could be storing the address with tp
pre-subtracted in the dtv. This would also be optimal for archs with
TLSDESC support, at the expense of an extra addition in legacy
__tls_get_addr access. On some archs it may even save a temp register
in the TLSDESC function.
Rich
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