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From: "fw at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/113874] GNU2 TLS descriptor calls do not follow psABI on
 x86_64-linux-gnu
Date: Mon, 12 Feb 2024 12:32:45 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D113874

--- Comment #20 from Florian Weimer <fw at gcc dot gnu.org> ---
(In reply to H.J. Lu from comment #19)
> (In reply to Florian Weimer from comment #9)
> > (In reply to H.J. Lu from comment #7)
> > > > The __tls_get_addr call with the default approach potentially needs=
 to solve
> > > > the same problem, doesn't it?
> > >=20
> > > Isn't __tls_get_addr called via the PLT entry?
> >=20
> > I'm not sure if that matters? Even if the lazy binding trampoline is ac=
tive,
> > it won't protect the actual call.
>=20
> Non-GNU2 TLS has
>=20
> 0000000000004000  0000000100000007 R_X86_64_JUMP_SLOT     0000000000000000
> __tls_get_addr + 1010
>=20
> which calls _dl_runtime_resolve with lazy binding. _dl_runtime_resolve
> preserves all caller-saved registers.

The dynamic linker preserves register contents during lazy binding and rest=
ores
them before calling __tls_get_addr, so it doesn't help with __tls_get_addr
register usage itself. And lazy binding happens only once per process and
object, while we need to protect the first call on every thread.=