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bug#61960: 30.0.50; Unexec build reliably crashes during loadup
From: |
Eli Zaretskii |
Subject: |
bug#61960: 30.0.50; Unexec build reliably crashes during loadup |
Date: |
Sun, 02 Jul 2023 08:52:18 +0300 |
> From: Konstantin Kharlamov <hi-angel@yandex.ru>
> Date: Sun, 02 Jul 2023 04:50:26 +0300
>
> I've found a diff that fixes the build, but whether it's okay is worth
> discussion:
>
> diff --git a/src/gmalloc.c b/src/gmalloc.c
> index e655d69f660..f49bb01e08b 100644
> --- a/src/gmalloc.c
> +++ b/src/gmalloc.c
> @@ -1704,7 +1704,7 @@ allocated_via_gmalloc (void *ptr)
> return false;
> size_t block = BLOCK (ptr);
> size_t blockmax = _heaplimit - 1;
> - return block <= blockmax && _heapinfo[block].busy.type != 0;
> + return block <= blockmax;
> }
>
> /* See the comments near the beginning of this file for explanations
>
> Here's what happens: Emacs uses internal stack-based allocator (apparently
> allocating
> with sbrk(), but I'm not sure) along with the system allocator. Whenever a
> memory is
> allocated from the internal allocator, you can't call `free()` on it.
>
> When Emacs wants to free memory, it calls `hybrid_free_1()`, which internally
> determines whether the `ptr` passed belongs to system heap or to Emacs
> stack. Determining in turn is done by `allocated_via_gmalloc()`.
>
> Emacs also keeps the lowest and highest boundary of this stack in variables
> `_heapbase` and `_heaplimit` accordingly (except the latter is measured in
> "blocks"). The code in diff `block <= blockmax` simply makes sure that the
> `ptr`
> passed is within the stack-allocated memory, which implies it can't be
> deallocated
> with `free()`
>
> There's a question though of the right-hand side that I remove, the
> `_heapinfo[block].busy.type != 0;`. Apparently the `type` should keep some
> memory
> info, and apparently there's a bug somewhere that screws it up. It is a bug
> worth
> fixing, although for some reason `rr replay` doesn't work for me with `temacs`
> (probably a bug in rr), and without reverse-execution tracking that down
> would be
> very hard.
>
> But I would argue that the right-hand side check has no value in this
> function,
> because to determine the source of allocation it's enough to just check
> whether `ptr`
> is in _heapbase .. _heaplimit range (barring the fact they're different
> units).
Thanks, but how do you explain that this code works as-is when the
BLOCK_ALIGN change is not used?