From: Brandon Casey <drafnel@gmail.com>
To: Junio C Hamano <gitster@pobox.com>
Cc: "Martin Ågren" <martin.agren@gmail.com>,
"git@vger.kernel.org" <git@vger.kernel.org>
Subject: Re: [PATCH v2 3/4] strbuf_setlen: don't write to strbuf_slopbuf
Date: Thu, 24 Aug 2017 11:29:13 -0700 [thread overview]
Message-ID: <CA+sFfMdYXDt2mgnWq-HQQyBsCqYZ+689BCKEOw7siGjQoUysjg@mail.gmail.com> (raw)
In-Reply-To: <xmqqpobkx610.fsf@gitster.mtv.corp.google.com>
On Thu, Aug 24, 2017 at 9:52 AM, Junio C Hamano <gitster@pobox.com> wrote:
> Brandon Casey <drafnel@gmail.com> writes:
>
>> Ah, you probably meant something like this:
>>
>> const char strbuf_slopbuf = '\0';
>>
>> which gcc will apparently place in the read-only segment. I did not know that.
>
> Yes but I highly suspect that it would be very compiler dependent
> and not something the language lawyers would recommend us to rely
> on.
I think compilers may have the option of placing variables that are
explicitly initialized to zero in the bss segment too, in addition to
those that are not explicitly initialized. So I agree that no one
should write code that depends on their variables being placed in one
segment or the other, but I could see someone using this behavior as
an additional safety check; kind of a free assert, aside from the
additional space in the .rodata segment.
> My response was primarily to answer "why?" with "because we did not
> bother". The above is a mere tangent, i.e. "multiple copies of
> empty strings is a horrible implementation (and there would be a way
> to do it with a single instance)".
Merely adding const to our current strbuf_slopbuf declaration does not
buy us anything since it will be allocated in r/w memory. i.e. it
would still be possible to modify it via the buf member of strbuf. So
you can't just do this:
const char strbuf_slopbuf[1];
That's pretty much equivalent to what we currently have since it only
restricts modifying the contents of strbuf_slopbuf directly through
the strbuf_slopbuf variable, but it does not restrict modifying it
through a pointer to that object.
Until yesterday, I was under the impression that the only way to
access data in the rodata segment was through a constant literal. So
my initial thought was that we could do something like:
const char * const strbuf_slopbuf = "";
..but that variable cannot be used in a static assignment like:
struct strbuf foo = {0, 0, (char*) strbuf_slopbuf};
So it seemed like our only option was to use a literal "" everywhere
instead of a slopbuf variable _if_ we wanted to have the guarantee
that our "slopbuf" could not be modified.
But what I learned yesterday, is that at least gcc/clang will place
the entire variable in the rodata segment if the variable is both
marked const _and_ initialized.
i.e. this will be allocated in the .rodata segment:
const char strbuf_slopbuf[1] = "";
>> #define STRBUF_INIT { .alloc = 0, .len = 0, .buf = (char*) &strbuf_slopbuf }
>>
>> respectively. Yeah, that's definitely preferable to a macro.
>> Something similar could be done in object.c.
>
> What is the main objective for doing this change? The "make sure we
> do not write into that slopbuf" assert() bothers you and you want to
> replace it with an address in the read-only segment?
Actually nothing about the patch bothers me. The point of that patch
is to make sure we don't accidentally modify the slopbuf. I was just
looking for a way for the compiler to help out and wondering if there
was a reason we didn't attempt to do so in the first place.
I think the main takeaway here is that I learned something yesterday
:-) I didn't actually intend to submit a patch for any of this, but
if anything useful came out of the discussion I thought Martin may
incorporate it into his patch if he wanted to.
-Brandon
next prev parent reply other threads:[~2017-08-24 18:29 UTC|newest]
Thread overview: 64+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-08-15 12:53 [PATCH/RFC 0/5] Some ThreadSanitizer-results Martin Ågren
2017-08-15 12:53 ` [PATCH 1/5] convert: initialize attr_action in convert_attrs Martin Ågren
2017-08-15 14:17 ` Torsten Bögershausen
2017-08-15 14:29 ` Torsten Bögershausen
2017-08-15 14:40 ` Martin Ågren
2017-08-15 12:53 ` [PATCH 2/5] pack-objects: take lock before accessing `remaining` Martin Ågren
2017-08-15 19:50 ` Johannes Sixt
2017-08-15 12:53 ` [PATCH 3/5] Makefile: define GIT_THREAD_SANITIZER Martin Ågren
2017-08-15 12:53 ` [PATCH 4/5] strbuf_reset: don't write to slopbuf with ThreadSanitizer Martin Ågren
2017-08-15 18:43 ` Junio C Hamano
2017-08-15 19:06 ` Martin Ågren
2017-08-15 19:19 ` Junio C Hamano
2017-08-15 12:53 ` [PATCH 5/5] ThreadSanitizer: add suppressions Martin Ågren
2017-08-15 12:53 ` tsan: t3008: hashmap_add touches size from multiple threads Martin Ågren
2017-08-15 17:59 ` Jeff Hostetler
2017-08-15 18:17 ` Stefan Beller
2017-08-15 18:40 ` Martin Ågren
2017-08-15 18:48 ` Stefan Beller
2017-08-15 19:21 ` Martin Ågren
2017-08-15 20:46 ` Jeff Hostetler
2017-08-30 18:59 ` [PATCH] hashmap: address ThreadSanitizer concerns Jeff Hostetler
2017-08-30 18:59 ` [PATCH] hashmap: add API to disable item counting when threaded Jeff Hostetler
2017-09-01 23:31 ` Johannes Schindelin
2017-09-01 23:50 ` Jonathan Nieder
2017-09-05 16:39 ` Jeff Hostetler
2017-09-05 17:13 ` Martin Ågren
2017-09-02 8:17 ` Jeff King
2017-09-04 15:59 ` Johannes Schindelin
2017-09-05 16:54 ` Jeff Hostetler
2017-09-06 3:43 ` Junio C Hamano
2017-09-05 16:33 ` Jeff Hostetler
2017-09-02 8:05 ` Jeff King
2017-09-05 17:07 ` Jeff Hostetler
2017-09-02 8:39 ` Simon Ruderich
2017-09-06 1:24 ` Junio C Hamano
2017-09-06 15:33 ` Jeff Hostetler
2017-09-06 15:43 ` [PATCH v2] hashmap: address ThreadSanitizer concerns Jeff Hostetler
2017-09-06 15:43 ` [PATCH v2] hashmap: add API to disable item counting when threaded Jeff Hostetler
2017-08-15 12:53 ` tsan: t5400: set_try_to_free_routine Martin Ågren
2017-08-15 17:35 ` Stefan Beller
2017-08-15 18:44 ` Martin Ågren
2017-08-17 10:57 ` Jeff King
2017-08-20 10:06 ` [PATCH/RFC 0/5] Some ThreadSanitizer-results Jeff King
2017-08-20 10:45 ` Martin Ågren
2017-08-21 17:43 ` [PATCH v2 0/4] " Martin Ågren
2017-08-21 17:43 ` [PATCH v2 1/4] convert: always initialize attr_action in convert_attrs Martin Ågren
2017-08-21 17:43 ` [PATCH v2 2/4] pack-objects: take lock before accessing `remaining` Martin Ågren
2017-08-21 17:43 ` [PATCH v2 3/4] strbuf_setlen: don't write to strbuf_slopbuf Martin Ågren
2017-08-23 17:24 ` Junio C Hamano
2017-08-23 17:43 ` Martin Ågren
2017-08-23 18:30 ` Junio C Hamano
2017-08-23 20:37 ` Brandon Casey
2017-08-23 21:04 ` Junio C Hamano
2017-08-23 21:20 ` Brandon Casey
2017-08-23 21:54 ` Brandon Casey
2017-08-23 22:11 ` Brandon Casey
2017-08-24 16:52 ` Junio C Hamano
2017-08-24 18:29 ` Brandon Casey [this message]
2017-08-24 19:16 ` Martin Ågren
2017-08-23 22:24 ` Junio C Hamano
2017-08-23 22:39 ` Brandon Casey
2017-08-21 17:43 ` [PATCH v2 4/4] ThreadSanitizer: add suppressions Martin Ågren
2017-08-25 17:04 ` Jeff King
2017-08-28 20:56 ` [PATCH v2 0/4] Some ThreadSanitizer-results Jeff Hostetler
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
List information: http://vger.kernel.org/majordomo-info.html
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=CA+sFfMdYXDt2mgnWq-HQQyBsCqYZ+689BCKEOw7siGjQoUysjg@mail.gmail.com \
--to=drafnel@gmail.com \
--cc=git@vger.kernel.org \
--cc=gitster@pobox.com \
--cc=martin.agren@gmail.com \
/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 public inbox
https://80x24.org/mirrors/git.git
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).