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bug#61960: 30.0.50; Unexec build reliably crashes during loadup
From: |
Konstantin Kharlamov |
Subject: |
bug#61960: 30.0.50; Unexec build reliably crashes during loadup |
Date: |
Sun, 02 Jul 2023 04:50:26 +0300 |
User-agent: |
Evolution 3.48.3 |
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).
- bug#61960: 30.0.50; Unexec build reliably crashes during loadup,
Konstantin Kharlamov <=