From: "Daniel Colascione" <dancol@dancol.org>
To: "Paul Eggert" <eggert@cs.ucla.edu>
Cc: Daniel Colascione <dancol@dancol.org>, emacs-devel@gnu.org
Subject: Re: [Emacs-diffs] master d826037 3/3: Remove the need for temacs.in
Date: Thu, 11 Apr 2019 15:24:38 -0700 [thread overview]
Message-ID: <e3d6d45ae527eff283e7cf9aa64522c7.squirrel@dancol.org> (raw)
In-Reply-To: <064a6c19-760d-a2b6-8f9c-8f7972557b65@cs.ucla.edu>
>> The linker deciding to lay out objects
>> within sections in a different order will break the dump.
>
> But why would the linker do that? It sounds like you're worrying that,
> even if
> we give the linker the same object files in the same order and ask it to
> link
> Emacs again, then the linker might generate a different executable, one
> that is
> incompatible with the previous one.
Who knows what linkers might do? Change is especially likely in an LTO
world when different linker flags or the linker binary changes? I want to
future-proof the fingerprint mechanism by fingerprinting something as
close as possible to the actual binary we run.
> But if that's the case, the temacs.in
> solution has the same problem so we're no worse off than before. And if
> it's not
> the case, then what exactly is the failure scenario here? I'm not seeing
> any
> failure scenarios for the current approach that can't also be failure
> scenarios
> for the previous approach, even now that I know that pdumper cares about
> object
> order within a section.
It's likely that the linker to lay out objects in a section identically in
two different builds when the only difference between the builds is the
content of an array. If we're worried about the array being folded into
the code, we can make it volatile. If someone changes a linker flag that
changes object ordering, temacs.in will change. If the linker gets
upgraded and flips the order of two variables in .data, temacs.in will
change.
>>> The mechanism could also "break" with whole-program optimization that
>>> inlines the fingerprint array, or with other plausible future changes
>>> to
>>> linkers as they get smarter.
>>
>> The optimization you've described doesn't matter: as long as changing
>> the
>> *value* of the fingerprint array (not its length!) preserves Emacs
>> *object
>> layout* change in Emacs, we're good.
>
> The optimization I describe could migrate some or all of the fingerprint
> array's
> contents into the code, with the amount of migration depending on the
> contents
> of the fingerprint array, and with the migrated contents omitted from the
> fingerprint array. I don't see how object layout (in the sense that you
> describe) would be preserved in that scenario.
Even if the array is migrated into code, changes to linker configuration
will still change the temacs.in fingerprint.
The invariant here isn't that temacs.in has to be same as temacs, but
rather that if temacs_1 differs from temacs_2 in ways that pdumper cares
about, then temacs.in_1 must have hash different from temacs.in_2. My
objection is that the object-hashing approach can result in a situation in
which temacs_1 and temacs_2 differ in ways that pdumper cares about but
nevertheless have the same fingerprint.
next prev parent reply other threads:[~2019-04-11 22:24 UTC|newest]
Thread overview: 24+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <20190409224339.20116.87667@vcs0.savannah.gnu.org>
[not found] ` <20190409224342.0DA1F20E54@vcs0.savannah.gnu.org>
2019-04-10 18:53 ` [Emacs-diffs] master d826037 3/3: Remove the need for temacs.in Daniel Colascione
2019-04-10 19:31 ` Paul Eggert
2019-04-10 19:42 ` Daniel Colascione
2019-04-10 20:43 ` Paul Eggert
2019-04-10 20:56 ` Daniel Colascione
2019-04-11 3:31 ` Paul Eggert
2019-04-11 22:24 ` Daniel Colascione [this message]
2019-04-12 3:45 ` Paul Eggert
2019-04-12 4:20 ` Daniel Colascione
2019-04-13 6:52 ` Eli Zaretskii
2019-04-14 3:40 ` Stefan Monnier
2019-04-14 3:43 ` Daniel Colascione
2019-04-14 4:08 ` Stefan Monnier
2019-04-14 14:03 ` Eli Zaretskii
2019-04-14 14:55 ` Stefan Monnier
2019-04-14 15:47 ` dancol
2019-04-14 17:30 ` Stefan Monnier
2019-04-14 17:44 ` Eli Zaretskii
2019-04-15 0:19 ` Paul Eggert
2019-04-11 19:35 ` Stefan Monnier
2019-04-11 22:15 ` Daniel Colascione
2019-04-11 23:37 ` Stefan Monnier
[not found] ` <20190409224341.BED1520E43@vcs0.savannah.gnu.org>
2019-04-10 19:00 ` [Emacs-diffs] master e44ff2d 2/3: Remove assumption of uint64_t etc. in portable code Daniel Colascione
2019-04-10 19:51 ` Paul Eggert
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=e3d6d45ae527eff283e7cf9aa64522c7.squirrel@dancol.org \
--to=dancol@dancol.org \
--cc=eggert@cs.ucla.edu \
--cc=emacs-devel@gnu.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
Code repositories for project(s) associated with this external index
https://git.savannah.gnu.org/cgit/emacs.git
https://git.savannah.gnu.org/cgit/emacs/org-mode.git
This is an external index of several public inboxes,
see mirroring instructions on how to clone and mirror
all data and code used by this external index.