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From: James Y Knight <jyknight@google.com>
To: baiyang <baiyang@gmail.com>
Cc: musl <musl@lists.openwall.com>, Florian Weimer <fweimer@redhat.com>
Subject: Re: Re: [musl] The heap memory performance (malloc/free/realloc) is significantly degraded in musl 1.2 (compared to 1.1)
Date: Mon, 19 Sep 2022 20:13:57 -0400	[thread overview]
Message-ID: <CAA2zVHr-4Za81Zs3WSDWpDg1rBhnb=k=EAP-2oAvLeY3btKviQ@mail.gmail.com> (raw)
In-Reply-To: <2022092001404698842815@gmail.com>

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On Mon, Sep 19, 2022 at 1:40 PM baiyang <baiyang@gmail.com> wrote:

> Hi James,
>
> I looked at the code of tcmalloc, but I didn't find any of the problems
> you mentioned in the implementation of malloc_usable_size (see:
> https://github.com/google/tcmalloc/blob/9179bb884848c30616667ba129bcf9afee114c32/tcmalloc/tcmalloc.cc#L1099
>  ).
>
> On the contrary, similar to musl, tcmalloc also directly uses the return
> value of malloc_usable_size in its realloc implementation to determine
> whether memory needs to be reallocated:
> https://github.com/google/tcmalloc/blob/9179bb884848c30616667ba129bcf9afee114c32/tcmalloc/tcmalloc.cc#L1499
>
> I think this is enough to show that the return value of malloc_usable_size
> in tcmalloc is accurate and reliable, otherwise its own realloc will
> cause a segment fault.
>

It returns an accurate value, but it is undefined behavior to actually
_use_ that memory. You need to first call realloc -- and when calling
realloc, you (probably...) need to access the new memory via the return
value of the realloc call, rather than the pointer you passed as input --
even if they happen to have the same integer representation. (Though
there's certainly plenty of debate/discussion regarding the exact workings
of, and advisability of, such 'pointer provenance" rules...). In any case,
calling the function inside the malloc implementation to implement realloc
is a different matter entirely from user code calling it.

And, note that just because something works for you today doesn't mean it's
correct or allowed. It just means that you've been able to get away with
it. So far. Until one day it stops working. That's the really fun part of
Undefined Behavior...

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  parent reply	other threads:[~2022-09-20  0:14 UTC|newest]

Thread overview: 68+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-09-19  7:53 baiyang
2022-09-19 11:08 ` Szabolcs Nagy
2022-09-19 12:36   ` Florian Weimer
2022-09-19 13:46     ` Rich Felker
2022-09-19 13:53       ` James Y Knight
2022-09-19 17:40         ` baiyang
2022-09-19 18:14           ` Szabolcs Nagy
2022-09-19 18:40             ` baiyang
2022-09-19 19:07             ` Gabriel Ravier
2022-09-19 19:21               ` Rich Felker
2022-09-19 21:02                 ` Gabriel Ravier
2022-09-19 21:47                   ` Rich Felker
2022-09-19 22:31                     ` Gabriel Ravier
2022-09-19 22:46                       ` baiyang
2022-09-19 20:46             ` Nat!
2022-09-20  8:51               ` Szabolcs Nagy
2022-09-20  0:13           ` James Y Knight [this message]
2022-09-20  0:25             ` baiyang
2022-09-20  0:38               ` Rich Felker
2022-09-20  0:47                 ` baiyang
2022-09-20  1:00                   ` Rich Felker
2022-09-20  1:18                     ` baiyang
2022-09-20  2:15                       ` Rich Felker
2022-09-20  2:35                         ` baiyang
2022-09-20  3:28                           ` Rich Felker
2022-09-20  3:53                             ` baiyang
2022-09-20  5:41                               ` Rich Felker
2022-09-20  5:56                                 ` baiyang
2022-09-20 12:16                                   ` Rich Felker
2022-09-20 17:21                                     ` baiyang
2022-09-20  8:33       ` Florian Weimer
2022-09-20 13:54         ` Siddhesh Poyarekar
2022-09-20 16:59           ` James Y Knight
2022-09-20 17:34             ` Szabolcs Nagy
2022-09-20 19:53               ` James Y Knight
2022-09-24  8:55               ` Fangrui Song
2022-09-20 17:39             ` baiyang
2022-09-20 18:12               ` Quentin Rameau
2022-09-20 18:19                 ` Rich Felker
2022-09-20 18:26                   ` Alexander Monakov
2022-09-20 18:35                     ` baiyang
2022-09-20 20:33                       ` Gabriel Ravier
2022-09-20 20:45                         ` baiyang
2022-09-21  8:42                           ` NRK
2022-09-20 18:37                     ` Quentin Rameau
2022-09-21 10:15                   ` [musl] " 王志强
2022-09-21 16:11                     ` [musl] " 王志强
2022-09-21 17:15                     ` [musl] " Rich Felker
2022-09-21 17:58                       ` Rich Felker
2022-09-22  3:34                         ` [musl] " 王志强
2022-09-22  9:10                           ` [musl] " 王志强
2022-09-22  9:39                             ` [musl] " 王志强
2022-09-20 17:28           ` baiyang
2022-09-20 17:44             ` Siddhesh Poyarekar
2022-10-10 14:13           ` Florian Weimer
2022-09-19 13:43 ` Rich Felker
2022-09-19 17:32   ` baiyang
2022-09-19 18:15     ` Rich Felker
2022-09-19 18:44       ` baiyang
2022-09-19 19:18         ` Rich Felker
2022-09-19 19:45           ` baiyang
2022-09-19 20:07             ` Rich Felker
2022-09-19 20:17               ` baiyang
2022-09-19 20:28                 ` Rich Felker
2022-09-19 20:38                   ` baiyang
2022-09-19 22:02                 ` Quentin Rameau
2022-09-19 20:17             ` Joakim Sindholt
2022-09-19 20:33               ` baiyang

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