From: "H.J. Lu" <hjl.tools@gmail.com>
To: Szabolcs Nagy <szabolcs.nagy@arm.com>
Cc: Adhemerval Zanella <adhemerval.zanella@linaro.org>,
GNU C Library <libc-alpha@sourceware.org>
Subject: Re: [PATCH v4 08/10] csu: Move static pie self relocation later [BZ #27072]
Date: Tue, 19 Jan 2021 07:32:49 -0800 [thread overview]
Message-ID: <CAMe9rOo0QJf6N2aOUSQ82CDJQg38VdLbpYxf3Xpp5ZC76U1jVg@mail.gmail.com> (raw)
In-Reply-To: <20210119152441.GB3445@arm.com>
On Tue, Jan 19, 2021 at 7:24 AM Szabolcs Nagy <szabolcs.nagy@arm.com> wrote:
>
> The 01/19/2021 06:48, H.J. Lu wrote:
> > On Tue, Jan 19, 2021 at 6:37 AM Adhemerval Zanella via Libc-alpha
> > <libc-alpha@sourceware.org> wrote:
> > >
> > >
> > >
> > > On 19/01/2021 11:35, Szabolcs Nagy wrote:
> > > > The 01/19/2021 11:07, Adhemerval Zanella wrote:
> > > >> On 18/01/2021 13:25, Szabolcs Nagy via Libc-alpha wrote:
> > > >>> IFUNC resolvers may depend on tunables and cpu feature setup so
> > > >>> move static pie self relocation after those.
> > > >>>
> > > >>> It is hard to guarantee that the ealy startup code does not rely
> > > >>> on relocations so this is a bit fragile. It would be more robust
> > > >>> to handle RELATIVE relocs early and only IRELATIVE relocs later,
> > > >>> but the current relocation processing code cannot do that.
> > > >>>
> > > >>> The early startup code before relocation processing includes
> > > >>>
> > > >>> _dl_aux_init (auxvec);
> > > >>> __libc_init_secure ();
> > > >>> __tunables_init (__environ);
> > > >>> ARCH_INIT_CPU_FEATURES ();
> > > >>>
> > > >>> These are simple enough that RELATIVE relocs can be avoided.
> > > >>>
> > > >>> __ehdr_start may require RELATIVE relocation so it was moved
> > > >>> later, fortunately ehdr and phdr are not used in the early code.
> > > >>>
> > > >>> Fixes bug 27072.
> > > >>
> > > >> LGTM, thanks.
> > > >>
> > > >> Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>
> > > >
> > > >
> > > > sigh, this is an old version of this patch, i made a
> > > > mistake putting the series together.
> > > >
> > > > the problem is that _dl_phdr is used in ARCH_SETUP_TLS
> > > > (to get the tls program headers) so the __ehdr_start
> > > > magic should be before that (this only matters if auxv
> > > > lacks AT_PHDR for some reason, which should not happen
> > > > normally on linux, so testing won't show the problem)
> > >
> > > By normally do you mean it might happen on a specific kernel version
> > > or is it architecture specific?
>
> i guess __ehdr_start symbol can be useful and with it
> glibc does not have to depend on auxv (which an elf
> loader like valgrind/qemu-user may get wrong)
>
> however it is only used as a fallback and on linux
> AT_PHDR is always expected to be present. (i don't
> know if this ever triggers)
Only used on Hurd?
> >
> > I think we can leave __ehdr_start ASIS since it doesn't need RELATIVE
> > relocation. I verified it by adding -Wl,-z,report-relative-reloc when building
> > elf/sln on x86.
>
> it needs relative reloc on aarch64: it can be an undefined weak
> symbol and that must be 0. a pc relative address computation
> cannot give 0 (unless linker does some instruction rewriting,
> but on aarch64 the address computation is multiple instructions
> that can be spread far apart). so yeah it needs a GOT entry and
> that will be either 0 or needs a RELATIVE reloc.
On x86, I converted load from GOT to simple LEA without RELATIVE
in this case. But this is an x86 specific optimization.
--
H.J.
next prev parent reply other threads:[~2021-01-19 15:33 UTC|newest]
Thread overview: 45+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-01-18 16:22 [PATCH v4 00/10] fix ifunc with static pie " Szabolcs Nagy
2021-01-18 16:23 ` [PATCH v4 01/10] configure: Require PI_STATIC_AND_HIDDEN for static pie Szabolcs Nagy
2021-01-18 16:23 ` [PATCH v4 02/10] libmvec: Add extra-test-objs to test-extras Szabolcs Nagy
2021-01-18 20:04 ` Adhemerval Zanella
2021-01-18 16:23 ` [PATCH v4 03/10] elf: Make the tunable struct definition internal only Szabolcs Nagy
2021-01-18 16:24 ` [PATCH v4 04/10] elf: Avoid RELATIVE relocs in __tunables_init Szabolcs Nagy
2021-01-18 16:24 ` [PATCH v4 05/10] Use hidden visibility for early static PIE code Szabolcs Nagy
2021-01-18 21:49 ` Adhemerval Zanella
2021-01-18 16:24 ` [PATCH v4 06/10] elf: Avoid RELATIVE relocation for _dl_sysinfo Szabolcs Nagy
2021-01-19 13:51 ` Adhemerval Zanella
2021-01-19 14:25 ` V2 " H.J. Lu
2021-01-19 14:35 ` Adhemerval Zanella
2021-01-18 16:25 ` [PATCH v4 07/10] Use <startup.h> in __libc_init_secure Szabolcs Nagy
2021-01-19 13:56 ` Adhemerval Zanella
2021-01-18 16:25 ` [PATCH v4 08/10] csu: Move static pie self relocation later [BZ #27072] Szabolcs Nagy
2021-01-19 14:07 ` Adhemerval Zanella
2021-01-19 14:35 ` Szabolcs Nagy
2021-01-19 14:36 ` Adhemerval Zanella
2021-01-19 14:48 ` H.J. Lu
2021-01-19 15:24 ` Szabolcs Nagy
2021-01-19 15:32 ` H.J. Lu [this message]
2021-01-19 16:47 ` H.J. Lu
2021-01-19 17:03 ` Szabolcs Nagy
2021-01-19 17:10 ` H.J. Lu
2021-01-19 17:25 ` Fāng-ruì Sòng
2021-01-19 17:33 ` H.J. Lu
2021-01-19 17:38 ` Fāng-ruì Sòng
2021-01-19 17:38 ` Szabolcs Nagy
2021-01-19 17:42 ` H.J. Lu
2021-01-19 17:47 ` Szabolcs Nagy
2021-01-19 17:53 ` H.J. Lu
2021-01-19 17:59 ` H.J. Lu
2021-01-18 16:25 ` [PATCH v4 09/10] x86: Check ifunc resolver with CPU_FEATURE_USABLE " Szabolcs Nagy
2021-01-19 14:11 ` Adhemerval Zanella
2021-01-19 14:37 ` V2 " H.J. Lu
2021-01-21 16:33 ` H.J. Lu
2021-01-18 16:26 ` [PATCH v4 10/10] Make libc symbols hidden in static PIE Szabolcs Nagy
2021-01-18 21:37 ` [PATCH v4 00/10] fix ifunc with static pie [BZ #27072] Adhemerval Zanella
2021-01-19 18:25 ` Szabolcs Nagy
2021-01-19 19:41 ` H.J. Lu
2021-01-19 20:16 ` Adhemerval Zanella
2021-01-19 21:38 ` H.J. Lu
2021-01-20 11:29 ` Adhemerval Zanella
2021-01-20 12:38 ` Szabolcs Nagy
2021-01-20 12:49 ` H.J. Lu
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