linux-kernel - [PATCH v2 2/2] mm: krealloc: clarify valid usage of __GFP

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20240812223707.32049-2-dakr@kernel.org>
Date: Tue, 13 Aug 2024 00:34:35 +0200
From: Danilo Krummrich <dakr@...nel.org>
To: akpm@...ux-foundation.org,
	cl@...ux.com,
	penberg@...nel.org,
	rientjes@...gle.com,
	iamjoonsoo.kim@....com,
	vbabka@...e.cz,
	roman.gushchin@...ux.dev,
	42.hyeyoo@...il.com
Cc: linux-kernel@...r.kernel.org,
	linux-mm@...ck.org,
	Danilo Krummrich <dakr@...nel.org>
Subject: [PATCH v2 2/2] mm: krealloc: clarify valid usage of __GFP_ZERO

Properly document that if __GFP_ZERO logic is requested, callers must
ensure that, starting with the initial memory allocation, every
subsequent call to this API for the same memory allocation is flagged
with __GFP_ZERO. Otherwise, it is possible that __GFP_ZERO is not fully
honored by this API.

Signed-off-by: Danilo Krummrich <dakr@...nel.org>
---
Changes in v2:
 - additionally to what we require callers to do, briefly explain what the
   implementation does (Andrew)
---
 include/linux/slab.h | 10 ++++++++++
 mm/slab_common.c     | 20 ++++++++++++++++++--
 2 files changed, 28 insertions(+), 2 deletions(-)

diff --git a/include/linux/slab.h b/include/linux/slab.h
index c9cb42203183..2282e67a01c7 100644
--- a/include/linux/slab.h
+++ b/include/linux/slab.h
@@ -733,6 +733,16 @@ static inline __alloc_size(1, 2) void *kmalloc_array_noprof(size_t n, size_t siz
  * @new_n: new number of elements to alloc
  * @new_size: new size of a single member of the array
  * @flags: the type of memory to allocate (see kmalloc)
+ *
+ * If __GFP_ZERO logic is requested, callers must ensure that, starting with the
+ * initial memory allocation, every subsequent call to this API for the same
+ * memory allocation is flagged with __GFP_ZERO. Otherwise, it is possible that
+ * __GFP_ZERO is not fully honored by this API.
+ *
+ * See krealloc_noprof() for further details.
+ *
+ * In any case, the contents of the object pointed to are preserved up to the
+ * lesser of the new and old sizes.
  */
 static inline __realloc_size(2, 3) void * __must_check krealloc_array_noprof(void *p,
 								       size_t new_n,
diff --git a/mm/slab_common.c b/mm/slab_common.c
index cff602cedf8e..1b380eb3b4f2 100644
--- a/mm/slab_common.c
+++ b/mm/slab_common.c
@@ -1301,11 +1301,27 @@ __do_krealloc(const void *p, size_t new_size, gfp_t flags)
  * @new_size: how many bytes of memory are required.
  * @flags: the type of memory to allocate.
  *
- * The contents of the object pointed to are preserved up to the
- * lesser of the new and old sizes (__GFP_ZERO flag is effectively ignored).
  * If @p is %NULL, krealloc() behaves exactly like kmalloc().  If @new_size
  * is 0 and @p is not a %NULL pointer, the object pointed to is freed.
  *
+ * If __GFP_ZERO logic is requested, callers must ensure that, starting with the
+ * initial memory allocation, every subsequent call to this API for the same
+ * memory allocation is flagged with __GFP_ZERO. Otherwise, it is possible that
+ * __GFP_ZERO is not fully honored by this API.
+ *
+ * This is the case, since krealloc() only knows about the bucket size of an
+ * allocation (but not the exact size it was allocated with) and hence
+ * implements the following semantics for shrinking and growing buffers with
+ * __GFP_ZERO.
+ *
+ *         new             bucket
+ * 0       size             size
+ * |--------|----------------|
+ * |  keep  |      zero      |
+ *
+ * In any case, the contents of the object pointed to are preserved up to the
+ * lesser of the new and old sizes.
+ *
  * Return: pointer to the allocated memory or %NULL in case of error
  */
 void *krealloc_noprof(const void *p, size_t new_size, gfp_t flags)
-- 
2.46.0