From: Sergey Bugaev <bugaevc@gmail.com>
To: Samuel Thibault <samuel.thibault@gnu.org>
Cc: bug-hurd@gnu.org, libc-alpha@sourceware.org,
"Flávio Cruz" <flaviocruz@gmail.com>
Subject: Re: [RFC PATCH 7/9] hurd: Generalize init-first.c to support x86_64
Date: Mon, 20 Feb 2023 20:10:29 +0300 [thread overview]
Message-ID: <CAN9u=HeqxTx44oNzvmHCge9=pBohVpk0j526bYtyMVeQUZM-Tw@mail.gmail.com> (raw)
In-Reply-To: <CAN9u=HfAOMiZtg3fgVMex7FhLXadwWDVVPxRedn2hNvMUahhUA@mail.gmail.com>
> I wish I had a better understanding of just what's going on in this
> file. Maybe a lot of the hacks can be rewritten in a nicer way. For
> instance, do we really need to hijack the return addresses and jump to
> init1 in this weird way, only to enable it to access argc/arg0? Since
> we know where they are on our stack (__builtin_frame_address (0) + 2
> or something like that), can't we just pass it a pointer?
>
> Let me actually try just that...
Eh, no, let's start even earlier than that. Please correct me if I
(inevitably) get things wrong.
_dl_init_first is Hurd-specific. It is called from the assembly code
in dl-machine.h, specifically there's a RTLD_START_SPECIAL_INIT macro
that's defined to call _dl_init_first on the Hurd, and to nothing
otherwise. This RTLD_START_SPECIAL_INIT is used ("invoked") in
i386/dl-machine.h, s390/s390-{32,64}/dl-machine.h, ia64/dl-machine.h,
and alpha/dl-machine.h (but notably not in x86_64/dl-machine.h). In
all cases, it's emphasized that "The special initializer gets called
with the stack just as the application's entry point will see it; it
can switch stacks if it moves these contents over."
But I conclude that:
- s390-gnu, ia64-gnu, and alpha-gnu ports are nonexistent, nor are
they realistically ever going to happen, so we can ignore them
completely
- the implementation does not seem to actually switch stacks (in fact,
I have removed the unused switch_stacks function in the last commit)
-- so the "gets called with the stack just as the application's entry
point will see it" property may not be important anymore?
The only thing it really needs, it seems, is a pointer to
argc/argv/envp & Hurd data block *somewhere*. It does not have to be
on the stack (though where else would it be), or immediately preceding
its call frame -- all that really matters is that there's a pointer.
So my thinking goes, why don't we just hook into _dl_start, which
already has this very pointer? And in fact, _dl_start calls
_dl_sysdep_start, for which there is already a Hurd version. Can't we
just call our logic from there, and not worry about the stack layout
and overwriting return addresses?
That would work for the SHARED case; we also need to do something for
the static case. In that case, we are invoked by static-start.S; do I
understand it right that the argc/argv/whatever is still located
on-stack even on x86_64 in this case, and not passed in registers
according to the calling convention? Then again, we should be able to
just use __builtin_frame_address (0) + 2 and avoid most of the hacks?
Please tell me if this makes any sense.
Sergey
next prev parent reply other threads:[~2023-02-20 17:10 UTC|newest]
Thread overview: 22+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-02-18 20:37 [RFC PATCH 0/9] More x86_64-gnu glibc work Sergey Bugaev
2023-02-18 20:37 ` [RFC PATCH 1/9] hurd: Move thread state manipulation into _hurd_tls_new () Sergey Bugaev
2023-02-19 23:32 ` Samuel Thibault
2023-02-18 20:37 ` [RFC PATCH 2/9] hurd: Use proper integer types Sergey Bugaev
2023-02-19 23:33 ` Samuel Thibault
2023-02-18 20:37 ` [RFC PATCH 3/9] hurd: Fix xattr function return type Sergey Bugaev
2023-02-19 23:34 ` Samuel Thibault
2023-02-18 20:37 ` [RFC PATCH 4/9] hurd: Make timer_t pointer-sized Sergey Bugaev
2023-02-19 23:35 ` Samuel Thibault
2023-02-18 20:37 ` [RFC PATCH 5/9] hurd: Simplify init-first.c a bit Sergey Bugaev
2023-02-19 23:45 ` Samuel Thibault
2023-02-18 20:37 ` [RFC PATCH 6/9] mach: Use PAGE_SIZE Sergey Bugaev
2023-02-19 23:47 ` Samuel Thibault
2023-02-18 20:37 ` [RFC PATCH 7/9] hurd: Generalize init-first.c to support x86_64 Sergey Bugaev
2023-02-20 0:01 ` Samuel Thibault
2023-02-20 16:16 ` Sergey Bugaev
2023-02-20 17:10 ` Sergey Bugaev [this message]
2023-02-20 17:31 ` Samuel Thibault
2023-02-20 17:27 ` Noah Goldstein
2023-02-18 20:37 ` [RFC PATCH 8/9 gnumach] Add i386_fsgs_base_state Sergey Bugaev
2023-02-18 20:37 ` [RFC PATCH 9/9] hurd, htl: Add some more x86_64-specific code Sergey Bugaev
2023-02-20 0:30 ` Samuel Thibault
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