[gnome-keyring/wip/smcv/allow-for-overhead: 1/2] egg-secure-memory: Allocate enough space for the guard pointers

[Simon McVittie <smcv src gnome org>]

commit ab88eeb947a6bb82875124e6fcaf67ece20ddd13
Author: Simon McVittie <smcv debian org>
Date:   Sun Sep 26 16:04:19 2021 +0100

    egg-secure-memory: Allocate enough space for the guard pointers
    
    Cell allocations have an overhead of two pointer-sized words, which we
    need to take into account. Otherwise, whenever we try to allocate a new
    block to hold at least `DEFAULT_BLOCK_SIZE - 2 * sizeof (void *)` bytes,
    it will end up too small to allocate a cell of that size and we will
    fall back to non-mlock'd memory with a warning.
    
    Fixes: 982aadd9 "Completely overhaul the secure memory allocator"
    Resolves: https://gitlab.gnome.org/GNOME/gnome-keyring/-/issues/36
    Signed-off-by: Simon McVittie <smcv debian org>

 egg/egg-secure-memory.c | 5 +++++
 1 file changed, 5 insertions(+)
---
diff --git a/egg/egg-secure-memory.c b/egg/egg-secure-memory.c
index ed6fbcc8..7391854d 100644
--- a/egg/egg-secure-memory.c
+++ b/egg/egg-secure-memory.c
@@ -959,6 +959,11 @@ sec_block_create (size_t size,
                return NULL;
        }
 
+       /* Whatever size we were asked for, we'll need a little more space:
+        * each allocation has an overhead of 2 words, for the guard
+        * pointers before and after. */
+       size += (2 * sizeof (word_t));
+
        /* The size above is a minimum, we're free to go bigger */
        if (size < DEFAULT_BLOCK_SIZE)
                size = DEFAULT_BLOCK_SIZE;