Drop flex_arrays (586187d7) · Commits · e / devices / android_kernel_fairphone_FP5

Documentation/core-api/flexible-arrays.rst

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		===================================
		Using flexible arrays in the kernel
		===================================

		Large contiguous memory allocations can be unreliable in the Linux kernel.
		Kernel programmers will sometimes respond to this problem by allocating
		pages with :c:func:`vmalloc()`. This solution not ideal, though. On 32-bit
		systems, memory from vmalloc() must be mapped into a relatively small address
		space; it's easy to run out. On SMP systems, the page table changes required
		by vmalloc() allocations can require expensive cross-processor interrupts on
		all CPUs. And, on all systems, use of space in the vmalloc() range increases
		pressure on the translation lookaside buffer (TLB), reducing the performance
		of the system.

		In many cases, the need for memory from vmalloc() can be eliminated by piecing
		together an array from smaller parts; the flexible array library exists to make
		this task easier.

		A flexible array holds an arbitrary (within limits) number of fixed-sized
		objects, accessed via an integer index. Sparse arrays are handled
		reasonably well. Only single-page allocations are made, so memory
		allocation failures should be relatively rare. The down sides are that the
		arrays cannot be indexed directly, individual object size cannot exceed the
		system page size, and putting data into a flexible array requires a copy
		operation. It's also worth noting that flexible arrays do no internal
		locking at all; if concurrent access to an array is possible, then the
		caller must arrange for appropriate mutual exclusion.

		The creation of a flexible array is done with :c:func:`flex_array_alloc()`::

		#include <linux/flex_array.h>

		struct flex_array *flex_array_alloc(int element_size,
		unsigned int total,
		gfp_t flags);

		The individual object size is provided by ``element_size``, while total is the
		maximum number of objects which can be stored in the array. The flags
		argument is passed directly to the internal memory allocation calls. With
		the current code, using flags to ask for high memory is likely to lead to
		notably unpleasant side effects.

		It is also possible to define flexible arrays at compile time with::

		DEFINE_FLEX_ARRAY(name, element_size, total);

		This macro will result in a definition of an array with the given name; the
		element size and total will be checked for validity at compile time.

		Storing data into a flexible array is accomplished with a call to
		:c:func:`flex_array_put()`::

		int flex_array_put(struct flex_array *array, unsigned int element_nr,
		void *src, gfp_t flags);

		This call will copy the data from src into the array, in the position
		indicated by ``element_nr`` (which must be less than the maximum specified when
		the array was created). If any memory allocations must be performed, flags
		will be used. The return value is zero on success, a negative error code
		otherwise.

		There might possibly be a need to store data into a flexible array while
		running in some sort of atomic context; in this situation, sleeping in the
		memory allocator would be a bad thing. That can be avoided by using
		``GFP_ATOMIC`` for the flags value, but, often, there is a better way. The
		trick is to ensure that any needed memory allocations are done before
		entering atomic context, using :c:func:`flex_array_prealloc()`::

		int flex_array_prealloc(struct flex_array *array, unsigned int start,
		unsigned int nr_elements, gfp_t flags);

		This function will ensure that memory for the elements indexed in the range
		defined by ``start`` and ``nr_elements`` has been allocated. Thereafter, a
		``flex_array_put()`` call on an element in that range is guaranteed not to
		block.

		Getting data back out of the array is done with :c:func:`flex_array_get()`::

		void flex_array_get(struct flex_array fa, unsigned int element_nr);

		The return value is a pointer to the data element, or NULL if that
		particular element has never been allocated.

		Note that it is possible to get back a valid pointer for an element which
		has never been stored in the array. Memory for array elements is allocated
		one page at a time; a single allocation could provide memory for several
		adjacent elements. Flexible array elements are normally initialized to the
		value ``FLEX_ARRAY_FREE`` (defined as 0x6c in <linux/poison.h>), so errors
		involving that number probably result from use of unstored array entries.
		Note that, if array elements are allocated with ``__GFP_ZERO``, they will be
		initialized to zero and this poisoning will not happen.

		Individual elements in the array can be cleared with
		:c:func:`flex_array_clear()`::

		int flex_array_clear(struct flex_array *array, unsigned int element_nr);

		This function will set the given element to ``FLEX_ARRAY_FREE`` and return
		zero. If storage for the indicated element is not allocated for the array,
		``flex_array_clear()`` will return ``-EINVAL`` instead. Note that clearing an
		element does not release the storage associated with it; to reduce the
		allocated size of an array, call :c:func:`flex_array_shrink()`::

		int flex_array_shrink(struct flex_array *array);

		The return value will be the number of pages of memory actually freed.
		This function works by scanning the array for pages containing nothing but
		``FLEX_ARRAY_FREE`` bytes, so (1) it can be expensive, and (2) it will not work
		if the array's pages are allocated with ``__GFP_ZERO``.

		It is possible to remove all elements of an array with a call to
		:c:func:`flex_array_free_parts()`::

		void flex_array_free_parts(struct flex_array *array);

		This call frees all elements, but leaves the array itself in place.
		Freeing the entire array is done with :c:func:`flex_array_free()`::

		void flex_array_free(struct flex_array *array);

		As of this writing, there are no users of flexible arrays in the mainline
		kernel. The functions described here are also not exported to modules;
		that will probably be fixed when somebody comes up with a need for it.


		Flexible array functions
		------------------------

		.. kernel-doc:: include/linux/flex_array.h

Documentation/flexible-arrays.txt

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		===================================
		Using flexible arrays in the kernel
		===================================

		:Updated: Last updated for 2.6.32
		:Author: Jonathan Corbet <corbet@lwn.net>

		Large contiguous memory allocations can be unreliable in the Linux kernel.
		Kernel programmers will sometimes respond to this problem by allocating
		pages with vmalloc(). This solution not ideal, though. On 32-bit systems,
		memory from vmalloc() must be mapped into a relatively small address space;
		it's easy to run out. On SMP systems, the page table changes required by
		vmalloc() allocations can require expensive cross-processor interrupts on
		all CPUs. And, on all systems, use of space in the vmalloc() range
		increases pressure on the translation lookaside buffer (TLB), reducing the
		performance of the system.

		In many cases, the need for memory from vmalloc() can be eliminated by
		piecing together an array from smaller parts; the flexible array library
		exists to make this task easier.

		A flexible array holds an arbitrary (within limits) number of fixed-sized
		objects, accessed via an integer index. Sparse arrays are handled
		reasonably well. Only single-page allocations are made, so memory
		allocation failures should be relatively rare. The down sides are that the
		arrays cannot be indexed directly, individual object size cannot exceed the
		system page size, and putting data into a flexible array requires a copy
		operation. It's also worth noting that flexible arrays do no internal
		locking at all; if concurrent access to an array is possible, then the
		caller must arrange for appropriate mutual exclusion.

		The creation of a flexible array is done with::

		#include <linux/flex_array.h>

		struct flex_array *flex_array_alloc(int element_size,
		unsigned int total,
		gfp_t flags);

		The individual object size is provided by element_size, while total is the
		maximum number of objects which can be stored in the array. The flags
		argument is passed directly to the internal memory allocation calls. With
		the current code, using flags to ask for high memory is likely to lead to
		notably unpleasant side effects.

		It is also possible to define flexible arrays at compile time with::

		DEFINE_FLEX_ARRAY(name, element_size, total);

		This macro will result in a definition of an array with the given name; the
		element size and total will be checked for validity at compile time.

		Storing data into a flexible array is accomplished with a call to::

		int flex_array_put(struct flex_array *array, unsigned int element_nr,
		void *src, gfp_t flags);

		This call will copy the data from src into the array, in the position
		indicated by element_nr (which must be less than the maximum specified when
		the array was created). If any memory allocations must be performed, flags
		will be used. The return value is zero on success, a negative error code
		otherwise.

		There might possibly be a need to store data into a flexible array while
		running in some sort of atomic context; in this situation, sleeping in the
		memory allocator would be a bad thing. That can be avoided by using
		GFP_ATOMIC for the flags value, but, often, there is a better way. The
		trick is to ensure that any needed memory allocations are done before
		entering atomic context, using::

		int flex_array_prealloc(struct flex_array *array, unsigned int start,
		unsigned int nr_elements, gfp_t flags);

		This function will ensure that memory for the elements indexed in the range
		defined by start and nr_elements has been allocated. Thereafter, a
		flex_array_put() call on an element in that range is guaranteed not to
		block.

		Getting data back out of the array is done with::

		void flex_array_get(struct flex_array fa, unsigned int element_nr);

		The return value is a pointer to the data element, or NULL if that
		particular element has never been allocated.

		Note that it is possible to get back a valid pointer for an element which
		has never been stored in the array. Memory for array elements is allocated
		one page at a time; a single allocation could provide memory for several
		adjacent elements. Flexible array elements are normally initialized to the
		value FLEX_ARRAY_FREE (defined as 0x6c in <linux/poison.h>), so errors
		involving that number probably result from use of unstored array entries.
		Note that, if array elements are allocated with __GFP_ZERO, they will be
		initialized to zero and this poisoning will not happen.

		Individual elements in the array can be cleared with::

		int flex_array_clear(struct flex_array *array, unsigned int element_nr);

		This function will set the given element to FLEX_ARRAY_FREE and return
		zero. If storage for the indicated element is not allocated for the array,
		flex_array_clear() will return -EINVAL instead. Note that clearing an
		element does not release the storage associated with it; to reduce the
		allocated size of an array, call::

		int flex_array_shrink(struct flex_array *array);

		The return value will be the number of pages of memory actually freed.
		This function works by scanning the array for pages containing nothing but
		FLEX_ARRAY_FREE bytes, so (1) it can be expensive, and (2) it will not work
		if the array's pages are allocated with __GFP_ZERO.

		It is possible to remove all elements of an array with a call to::

		void flex_array_free_parts(struct flex_array *array);

		This call frees all elements, but leaves the array itself in place.
		Freeing the entire array is done with::

		void flex_array_free(struct flex_array *array);

		As of this writing, there are no users of flexible arrays in the mainline
		kernel. The functions described here are also not exported to modules;
		that will probably be fixed when somebody comes up with a need for it.

include/linux/flex_array.h

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		/* SPDX-License-Identifier: GPL-2.0 */
		#ifndef _FLEX_ARRAY_H
		#define _FLEX_ARRAY_H

		#include <linux/types.h>
		#include <linux/reciprocal_div.h>
		#include <asm/page.h>

		#define FLEX_ARRAY_PART_SIZE PAGE_SIZE
		#define FLEX_ARRAY_BASE_SIZE PAGE_SIZE

		struct flex_array_part;

		/*
		* This is meant to replace cases where an array-like
		* structure has gotten too big to fit into kmalloc()
		* and the developer is getting tempted to use
		* vmalloc().
		*/

		struct flex_array {
		union {
		struct {
		int element_size;
		int total_nr_elements;
		int elems_per_part;
		struct reciprocal_value reciprocal_elems;
		struct flex_array_part *parts[];
		};
		/*
		* This little trick makes sure that
		* sizeof(flex_array) == PAGE_SIZE
		*/
		char padding[FLEX_ARRAY_BASE_SIZE];
		};
		};

		/* Number of bytes left in base struct flex_array, excluding metadata */
		#define FLEX_ARRAY_BASE_BYTES_LEFT \
		(FLEX_ARRAY_BASE_SIZE - offsetof(struct flex_array, parts))

		/* Number of pointers in base to struct flex_array_part pages */
		#define FLEX_ARRAY_NR_BASE_PTRS \
		(FLEX_ARRAY_BASE_BYTES_LEFT / sizeof(struct flex_array_part *))

		/* Number of elements of size that fit in struct flex_array_part */
		#define FLEX_ARRAY_ELEMENTS_PER_PART(size) \
		(FLEX_ARRAY_PART_SIZE / size)

		/*
		* Defines a statically allocated flex array and ensures its parameters are
		* valid.
		*/
		#define DEFINE_FLEX_ARRAY(__arrayname, __element_size, __total) \
		struct flex_array __arrayname = { { { \
		.element_size = (__element_size), \
		.total_nr_elements = (__total), \
		} } }; \
		static inline void __arrayname##_invalid_parameter(void) \
		{ \
		BUILD_BUG_ON((__total) > FLEX_ARRAY_NR_BASE_PTRS * \
		FLEX_ARRAY_ELEMENTS_PER_PART(__element_size)); \
		}

		/**
		* flex_array_alloc() - Creates a flexible array.
		* @element_size: individual object size.
		* @total: maximum number of objects which can be stored.
		* @flags: GFP flags
		*
		* Return: Returns an object of structure flex_array.
		*/
		struct flex_array *flex_array_alloc(int element_size, unsigned int total,
		gfp_t flags);

		/**
		* flex_array_prealloc() - Ensures that memory for the elements indexed in the
		* range defined by start and nr_elements has been allocated.
		* @fa: array to allocate memory to.
		* @start: start address
		* @nr_elements: number of elements to be allocated.
		* @flags: GFP flags
		*
		*/
		int flex_array_prealloc(struct flex_array *fa, unsigned int start,
		unsigned int nr_elements, gfp_t flags);

		/**
		* flex_array_free() - Removes all elements of a flexible array.
		* @fa: array to be freed.
		*/
		void flex_array_free(struct flex_array *fa);

		/**
		* flex_array_free_parts() - Removes all elements of a flexible array, but
		* leaves the array itself in place.
		* @fa: array to be emptied.
		*/
		void flex_array_free_parts(struct flex_array *fa);

		/**
		* flex_array_put() - Stores data into a flexible array.
		* @fa: array where element is to be stored.
		* @element_nr: position to copy, must be less than the maximum specified when
		* the array was created.
		* @src: data source to be copied into the array.
		* @flags: GFP flags
		*
		* Return: Returns zero on success, a negative error code otherwise.
		*/
		int flex_array_put(struct flex_array fa, unsigned int element_nr, void src,
		gfp_t flags);

		/**
		* flex_array_clear() - Clears an individual element in the array, sets the
		* given element to FLEX_ARRAY_FREE.
		* @element_nr: element position to clear.
		* @fa: array to which element to be cleared belongs.
		*
		* Return: Returns zero on success, -EINVAL otherwise.
		*/
		int flex_array_clear(struct flex_array *fa, unsigned int element_nr);

		/**
		* flex_array_get() - Retrieves data into a flexible array.
		*
		* @element_nr: Element position to retrieve data from.
		* @fa: array from which data is to be retrieved.
		*
		* Return: Returns a pointer to the data element, or NULL if that
		* particular element has never been allocated.
		*/
		void flex_array_get(struct flex_array fa, unsigned int element_nr);

		/**
		* flex_array_shrink() - Reduces the allocated size of an array.
		* @fa: array to shrink.
		*
		* Return: Returns number of pages of memory actually freed.
		*
		*/
		int flex_array_shrink(struct flex_array *fa);

		#define flex_array_put_ptr(fa, nr, src, gfp) \
		flex_array_put(fa, nr, (void *)&(src), gfp)

		void flex_array_get_ptr(struct flex_array fa, unsigned int element_nr);

		#endif /* _FLEX_ARRAY_H */

include/linux/poison.h

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		@@ -83,9 +83,6 @@
		#define MUTEX_DEBUG_FREE 0x22
		#define MUTEX_POISON_WW_CTX ((void *) 0x500 + POISON_POINTER_DELTA)

		/******** lib/flex_array.c ********/
		#define FLEX_ARRAY_FREE 0x6c /* for use-after-free poisoning */

		/******** security/ ********/
		#define KEY_DESTROY 0xbd

lib/Makefile

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Original line number	Diff line number	Diff line
		@@ -35,7 +35,7 @@ obj-y += lockref.o

		obj-y += bcd.o div64.o sort.o parser.o debug_locks.o random32.o \
		bust_spinlocks.o kasprintf.o bitmap.o scatterlist.o \
		gcd.o lcm.o list_sort.o uuid.o flex_array.o iov_iter.o clz_ctz.o \
		gcd.o lcm.o list_sort.o uuid.o iov_iter.o clz_ctz.o \
		bsearch.o find_bit.o llist.o memweight.o kfifo.o \
		percpu-refcount.o rhashtable.o reciprocal_div.o \
		once.o refcount.o usercopy.o errseq.o bucket_locks.o \