[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Message-ID: <CAN30aBGKpn_KMe+zn_OTb6RhDyA5hXH+yg_BmKdh2VDc8znO9Q@mail.gmail.com>
Date: Sat, 30 Nov 2024 09:51:24 -0800
From: Fangrui Song <i@...kray.me>
To: musl@...ts.openwall.com
Cc: Alex Rønne Petersen <alex@...xrp.com>,
Alexander Monakov <amonakov@...ras.ru>
Subject: Re: [PATCH] s390x: Mark __tls_get_addr hidden before invoking it.
On Fri, Nov 29, 2024 at 7:20 PM Rich Felker <dalias@...c.org> wrote:
>
> 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
(I am not versed in s390x assembly, but I have some notes about __tls_get_offset
https://maskray.me/blog/2024-02-11-toolchain-notes-on-z-architecture#general-dynamic-tls-model
The 32-bit ABI had to use __tls_get_offset because some nice
general-instructions-extension was unavailable when the ABI was
codified.
The 64-bit ABI following the 32-bit __tls_get_offset was just unfortunate.
Powered by blists - more mailing lists
Confused about mailing lists and their use?
Read about mailing lists on Wikipedia
and check out these
guidelines on proper formatting of your messages.
