On 12/4/25 00:30, Kees Cook wrote:
> As done for kmalloc_obj*(), introduce a type-aware allocator for flexible
> arrays, which may also have "counted_by" annotations:
>
> ptr = kmalloc(struct_size(ptr, flex_member, count), gfp);
>
> becomes:
>
> ptr = kmalloc_flex(*ptr, flex_member, count, gfp);
>
> The internal use of __flex_counter() allows for automatically setting
> the counter member of a struct's flexible array member when it has
> been annotated with __counted_by(), avoiding any missed early size
> initializations while __counted_by() annotations are added to the
> kernel. Additionally, this also checks for "too large" allocations based
> on the type size of the counter variable. For example:
>
> if (count > type_max(ptr->flex_counter))
> fail...;
> size = struct_size(ptr, flex_member, count);
> ptr = kmalloc(size, gfp);
> ptr->flex_counter = count;
>
> becomes (n.b. unchanged from earlier example):
>
> ptr = kmalloc_flex(*ptr, flex_member, count, gfp);
> ptr->flex_count = count;
^ flex_counter ?
But if it was "too large", ptr is NULL so this will oops?
> Note that manual initialization of the flexible array counter is still
> required (at some point) after allocation as not all compiler versions
> support the __counted_by annotation yet. But doing it internally makes
> sure they cannot be missed when __counted_by _is_ available, meaning
> that the bounds checker will not trip due to the lack of "early enough"
> initializations that used to work before enabling the stricter bounds
> checking. For example:
>
> ptr = kmalloc_flex(*ptr, flex_member, count, gfp);
> fill(ptr->flex, count);
> ptr->flex_count = count;
>
> This works correctly before adding a __counted_by annotation (since
> nothing is checking ptr->flex accesses against ptr->flex_count). After
> adding the annotation, the bounds sanitizer would trip during fill()
> because ptr->flex_count wasn't set yet. But with kmalloc_flex() setting
> ptr->flex_count internally at allocation time, the existing code works
> without needing to move the ptr->flex_count assignment before the call
> to fill(). (This has been a stumbling block for __counted_by adoption.)
>
> Signed-off-by: Kees Cook <[email protected]>
> ---
> Cc: Jonathan Corbet <[email protected]>
> Cc: Vlastimil Babka <[email protected]>
> Cc: Andrew Morton <[email protected]>
> Cc: Christoph Lameter <[email protected]>
> Cc: David Rientjes <[email protected]>
> Cc: Roman Gushchin <[email protected]>
> Cc: Harry Yoo <[email protected]>
> Cc: "Gustavo A. R. Silva" <[email protected]>
> Cc: <[email protected]>
> Cc: <[email protected]>
> Cc: <[email protected]>
> Cc: <[email protected]>
> ---
> Documentation/process/deprecated.rst | 7 ++++
> include/linux/slab.h | 48 ++++++++++++++++++++++++++++
> 2 files changed, 55 insertions(+)
>
> diff --git a/Documentation/process/deprecated.rst
> b/Documentation/process/deprecated.rst
> index 91c628fa2d59..fed56864d036 100644
> --- a/Documentation/process/deprecated.rst
> +++ b/Documentation/process/deprecated.rst
> @@ -387,6 +387,7 @@ allocations. For example, these open coded assignments::
> ptr = kzalloc(sizeof(*ptr), gfp);
> ptr = kmalloc_array(count, sizeof(*ptr), gfp);
> ptr = kcalloc(count, sizeof(*ptr), gfp);
> + ptr = kmalloc(struct_size(ptr, flex_member, count), gfp);
> ptr = kmalloc(sizeof(struct foo, gfp);
>
> become, respectively::
> @@ -395,4 +396,10 @@ become, respectively::
> ptr = kzalloc_obj(*ptr, gfp);
> ptr = kmalloc_objs(*ptr, count, gfp);
> ptr = kzalloc_objs(*ptr, count, gfp);
> + ptr = kmalloc_flex(*ptr, flex_member, count, gfp);
> __auto_type ptr = kmalloc_obj(struct foo, gfp);
> +
> +If `ptr->flex_member` is annotated with __counted_by(), the allocation
> +will automatically fail if `count` is larger than the maximum
> +representable value that can be stored in the counter member associated
> +with `flex_member`.
> diff --git a/include/linux/slab.h b/include/linux/slab.h
> index 726457daedbd..2656ea610b68 100644
> --- a/include/linux/slab.h
> +++ b/include/linux/slab.h
> @@ -982,6 +982,33 @@ void *kmalloc_nolock_noprof(size_t size, gfp_t
> gfp_flags, int node);
> (TYPE *)KMALLOC(__obj_size, GFP); \
> })
>
> +/**
> + * __alloc_flex - Allocate an object that has a trailing flexible array
> + * @KMALLOC: kmalloc wrapper function to use for allocation.
> + * @GFP: GFP flags for the allocation.
> + * @TYPE: type of structure to allocate space for.
> + * @FAM: The name of the flexible array member of @TYPE structure.
> + * @COUNT: how many @FAM elements to allocate space for.
> + *
> + * Returns: Newly allocated pointer to @TYPE with @COUNT-many trailing
> + * @FAM elements, or NULL on failure or if @COUNT cannot be represented
> + * by the member of @TYPE that counts the @FAM elements (annotated via
> + * __counted_by()).
> + */
> +#define __alloc_flex(KMALLOC, GFP, TYPE, FAM, COUNT) \
> +({ \
> + const size_t __count = (COUNT); \
> + const size_t __obj_size = struct_size_t(TYPE, FAM, __count); \
> + TYPE *__obj_ptr; \
> + if (WARN_ON_ONCE(overflows_flex_counter_type(TYPE, FAM, __count))) \
> + __obj_ptr = NULL; \
> + else \
> + __obj_ptr = KMALLOC(__obj_size, GFP); \
> + if (__obj_ptr) \
> + __set_flex_counter(__obj_ptr->FAM, __count); \
> + __obj_ptr; \
> +})
> +
> /**
> * kmalloc_obj - Allocate a single instance of the given type
> * @VAR_OR_TYPE: Variable or type to allocate.
> @@ -1005,23 +1032,44 @@ void *kmalloc_nolock_noprof(size_t size, gfp_t
> gfp_flags, int node);
> #define kmalloc_objs(VAR_OR_TYPE, COUNT, GFP) \
> __alloc_objs(kmalloc, GFP, typeof(VAR_OR_TYPE), COUNT)
>
> +/**
> + * kmalloc_flex - Allocate a single instance of the given flexible structure
> + * @VAR_OR_TYPE: Variable or type to allocate (with its flex array).
> + * @FAM: The name of the flexible array member of the structure.
> + * @COUNT: How many flexible array member elements are desired.
> + * @GFP: GFP flags for the allocation.
> + *
> + * Returns: newly allocated pointer to @VAR_OR_TYPE on success, NULL on
> + * failure. If @FAM has been annotated with __counted_by(), the allocation
> + * will immediately fail if @COUNT is larger than what the type of the
> + * struct's counter variable can represent.
> + */
> +#define kmalloc_flex(VAR_OR_TYPE, FAM, COUNT, GFP) \
> + __alloc_flex(kmalloc, GFP, typeof(VAR_OR_TYPE), FAM, COUNT)
> +
> /* All kzalloc aliases for kmalloc_(obj|objs|flex). */
> #define kzalloc_obj(P, GFP) \
> __alloc_objs(kzalloc, GFP, typeof(P), 1)
> #define kzalloc_objs(P, COUNT, GFP) \
> __alloc_objs(kzalloc, GFP, typeof(P), COUNT)
> +#define kzalloc_flex(P, FAM, COUNT, GFP) \
> + __alloc_flex(kzalloc, GFP, typeof(P), FAM, COUNT)
>
> /* All kvmalloc aliases for kmalloc_(obj|objs|flex). */
> #define kvmalloc_obj(P, GFP) \
> __alloc_objs(kvmalloc, GFP, typeof(P), 1)
> #define kvmalloc_objs(P, COUNT, GFP) \
> __alloc_objs(kvmalloc, GFP, typeof(P), COUNT)
> +#define kvmalloc_flex(P, FAM, COUNT, GFP) \
> + __alloc_flex(kvmalloc, GFP, typeof(P), FAM, COUNT)
>
> /* All kvzalloc aliases for kmalloc_(obj|objs|flex). */
> #define kvzalloc_obj(P, GFP) \
> __alloc_objs(kvzalloc, GFP, typeof(P), 1)
> #define kvzalloc_objs(P, COUNT, GFP) \
> __alloc_objs(kvzalloc, GFP, typeof(P), COUNT)
> +#define kvzalloc_flex(P, FAM, COUNT, GFP) \
> + __alloc_flex(kvzalloc, GFP, typeof(P), FAM, COUNT)
>
> #define kmem_buckets_alloc(_b, _size, _flags) \
> alloc_hooks(__kmalloc_node_noprof(PASS_BUCKET_PARAMS(_size, _b),
> _flags, NUMA_NO_NODE))
