Linux kmalloc doesn't return NULL on 0-byte allocations, but returns a fixed ZERO_PSIZE_PTR constant, which triggers a NULL page exception when dereferenced.
However, in barebox, allocator behavior differs: - glibc returns an unique pointer, but the standard also allows libc implementations to return NULL - tlsf returns NULL - dlmalloc returns a unique pointer - dummy malloc returns a unique pointer Let's unify this discrepancy by doing something completely else and returning a non-NULL non-unique pointer constant like Linux does. The benefit of that is: - NULL return value always means out-of-memory - Accessing a zero sized allocation triggers a NULL page exception This was noticed during porting of 9p2000 file system from Linux, which does variable sized allocations and can end up allocating 0 bytes, but expects that the allocation does not return NULL. Signed-off-by: Ahmad Fatoum <[email protected]> --- arch/sandbox/os/libc_malloc.c | 25 ++++++++++++++++++++++++- common/calloc.c | 2 +- common/dlmalloc.c | 16 +++++++++------- common/dummy_malloc.c | 3 +++ common/tlsf.c | 15 ++++++++++----- common/tlsf_malloc.c | 9 --------- include/malloc.h | 13 +++++++++++++ test/self/malloc.c | 3 ++- 8 files changed, 62 insertions(+), 24 deletions(-) diff --git a/arch/sandbox/os/libc_malloc.c b/arch/sandbox/os/libc_malloc.c index dd0f8670ff71..bb4fb1c9ead4 100644 --- a/arch/sandbox/os/libc_malloc.c +++ b/arch/sandbox/os/libc_malloc.c @@ -6,6 +6,10 @@ #include <stdlib.h> #include <malloc.h> +#define ZERO_SIZE_PTR ((void *)16) + +#define ZERO_OR_NULL_PTR(x) ((unsigned long)(x) <= \ + (unsigned long)ZERO_SIZE_PTR) #define BAREBOX_ENOMEM 12 #define BAREBOX_MALLOC_MAX_SIZE 0x40000000 @@ -19,6 +23,9 @@ void *barebox_memalign(size_t alignment, size_t bytes) { void *mem = NULL; + if (!bytes) + return ZERO_SIZE_PTR; + if (alignment <= BAREBOX_MALLOC_MAX_SIZE && bytes <= BAREBOX_MALLOC_MAX_SIZE) mem = memalign(alignment, bytes); if (!mem) @@ -29,9 +36,11 @@ void *barebox_memalign(size_t alignment, size_t bytes) void *barebox_malloc(size_t size) { - void *mem = NULL; + if (!size) + return ZERO_SIZE_PTR; + if (size <= BAREBOX_MALLOC_MAX_SIZE) mem = malloc(size); if (!mem) @@ -42,11 +51,15 @@ void *barebox_malloc(size_t size) size_t barebox_malloc_usable_size(void *mem) { + if (ZERO_OR_NULL_PTR(mem)) + return 0; return malloc_usable_size(mem); } void barebox_free(void *ptr) { + if (ZERO_OR_NULL_PTR(ptr)) + return; free(ptr); } @@ -54,6 +67,13 @@ void *barebox_realloc(void *ptr, size_t size) { void *mem = NULL; + if (!size) { + barebox_free(ptr); + return ZERO_SIZE_PTR; + } + if (ZERO_OR_NULL_PTR(ptr)) + ptr = NULL; + if (size <= BAREBOX_MALLOC_MAX_SIZE) mem = realloc(ptr, size); if (!mem) @@ -67,6 +87,9 @@ void *barebox_calloc(size_t n, size_t elem_size) size_t product; void *mem = NULL; + if (!n || !elem_size) + return ZERO_SIZE_PTR; + if (!__builtin_mul_overflow(n, elem_size, &product) && product <= BAREBOX_MALLOC_MAX_SIZE) mem = calloc(n, elem_size); diff --git a/common/calloc.c b/common/calloc.c index 17cbd9beefee..ee9322509751 100644 --- a/common/calloc.c +++ b/common/calloc.c @@ -13,7 +13,7 @@ void *calloc(size_t n, size_t elem_size) size_t size = size_mul(elem_size, n); void *r = malloc(size); - if (r && !want_init_on_alloc()) + if (!ZERO_OR_NULL_PTR(r) && !want_init_on_alloc()) memset(r, 0x0, size); return r; diff --git a/common/dlmalloc.c b/common/dlmalloc.c index 2b5723e127c4..c8e05ac68f2a 100644 --- a/common/dlmalloc.c +++ b/common/dlmalloc.c @@ -389,7 +389,7 @@ returns a unique pointer for malloc(0), so does realloc(p, 0). */ -/* #define REALLOC_ZERO_BYTES_FREES */ +#define REALLOC_ZERO_BYTES_FREES /* Define HAVE_MMAP to optionally make malloc() use mmap() to @@ -1165,6 +1165,8 @@ void *dlmalloc(size_t bytes) if (bytes > MALLOC_MAX_SIZE) { errno = ENOMEM; return NULL; + } else if (bytes == 0) { + return ZERO_SIZE_PTR; } nb = request2size(bytes); /* padded request size; */ @@ -1363,7 +1365,7 @@ void dlfree(void *mem) mchunkptr fwd; /* misc temp for linking */ int islr; /* track whether merging with last_remainder */ - if (!mem) /* free(0) has no effect */ + if (ZERO_OR_NULL_PTR(mem)) /* free(0) has no effect */ return; p = mem2chunk(mem); @@ -1431,7 +1433,7 @@ size_t dlmalloc_usable_size(void *mem) { mchunkptr p; - if (!mem) + if (ZERO_OR_NULL_PTR(mem)) return 0; p = mem2chunk(mem); @@ -1495,7 +1497,7 @@ void *dlrealloc(void *oldmem, size_t bytes) #ifdef REALLOC_ZERO_BYTES_FREES if (bytes == 0) { dlfree(oldmem); - return NULL; + return ZERO_SIZE_PTR; } #endif @@ -1505,7 +1507,7 @@ void *dlrealloc(void *oldmem, size_t bytes) } /* realloc of null is supposed to be same as malloc */ - if (!oldmem) + if (ZERO_OR_NULL_PTR(oldmem)) return dlmalloc(bytes); newp = oldp = mem2chunk(oldmem); @@ -1760,8 +1762,8 @@ void *dlcalloc(size_t n, size_t elem_size) mem = dlmalloc(sz); - if (!mem) - return NULL; + if (ZERO_OR_NULL_PTR(mem)) + return mem; else { p = mem2chunk(mem); diff --git a/common/dummy_malloc.c b/common/dummy_malloc.c index 4973f35650ab..956a54da0396 100644 --- a/common/dummy_malloc.c +++ b/common/dummy_malloc.c @@ -13,6 +13,9 @@ void *memalign(size_t alignment, size_t bytes) { void *mem = NULL; + if (!bytes) + return ZERO_SIZE_PTR; + if (alignment <= MALLOC_MAX_SIZE && bytes <= MALLOC_MAX_SIZE) mem = sbrk(bytes + alignment); if (!mem) { diff --git a/common/tlsf.c b/common/tlsf.c index 146f1b7ebe81..5504453a9453 100644 --- a/common/tlsf.c +++ b/common/tlsf.c @@ -743,7 +743,7 @@ void tlsf_walk_pool(pool_t pool, tlsf_walker walker, void* user) size_t tlsf_block_size(void* ptr) { size_t size = 0; - if (ptr) + if (likely(!ZERO_OR_NULL_PTR(ptr))) { const block_header_t* block = block_from_ptr(ptr); size = block_size(block); @@ -935,6 +935,8 @@ void* tlsf_malloc(tlsf_t tlsf, size_t size) const size_t adjust = adjust_request_size(size, ALIGN_SIZE); block_header_t* block; + if (!size) + return ZERO_SIZE_PTR; if (!adjust) return NULL; @@ -961,12 +963,14 @@ void* tlsf_memalign(tlsf_t tlsf, size_t align, size_t size) /* ** If alignment is less than or equals base alignment, we're done. - ** If we requested 0 bytes, return null, as tlsf_malloc(0) does. + ** If we requested 0 bytes, return ZERO_SIZE_PTR, as tlsf_malloc(0) does. */ const size_t aligned_size = (adjust && align > ALIGN_SIZE) ? size_with_gap : adjust; block_header_t* block; + if (!size) + return ZERO_SIZE_PTR; if (!adjust || !size_with_gap) return NULL; @@ -1008,7 +1012,7 @@ void* tlsf_memalign(tlsf_t tlsf, size_t align, size_t size) void tlsf_free(tlsf_t tlsf, void* ptr) { /* Don't attempt to free a NULL pointer. */ - if (ptr) + if (!ZERO_OR_NULL_PTR(ptr)) { control_t* control = tlsf_cast(control_t*, tlsf); block_header_t* block = block_from_ptr(ptr); @@ -1042,12 +1046,13 @@ void* tlsf_realloc(tlsf_t tlsf, void* ptr, size_t size) void* p = 0; /* Zero-size requests are treated as free. */ - if (ptr && size == 0) + if (size == 0) { tlsf_free(tlsf, ptr); + return ZERO_SIZE_PTR; } /* Requests with NULL pointers are treated as malloc. */ - else if (!ptr) + else if (ZERO_OR_NULL_PTR(ptr)) { p = tlsf_malloc(tlsf, size); } diff --git a/common/tlsf_malloc.c b/common/tlsf_malloc.c index 1b9d9c7702c2..6a90ee47199f 100644 --- a/common/tlsf_malloc.c +++ b/common/tlsf_malloc.c @@ -17,12 +17,6 @@ extern tlsf_t tlsf_mem_pool; void *malloc(size_t bytes) { void *mem; - /* - * tlsf_malloc returns NULL for zero bytes, we instead want - * to have a valid pointer. - */ - if (!bytes) - bytes = 1; mem = tlsf_malloc(tlsf_mem_pool, bytes); if (!mem) @@ -40,9 +34,6 @@ EXPORT_SYMBOL(free); size_t malloc_usable_size(void *mem) { - if (!mem) - return 0; - return tlsf_block_size(mem); } EXPORT_SYMBOL(malloc_usable_size); diff --git a/include/malloc.h b/include/malloc.h index c38726342c08..0b74746360c0 100644 --- a/include/malloc.h +++ b/include/malloc.h @@ -8,6 +8,19 @@ #define MALLOC_SHIFT_MAX 30 #define MALLOC_MAX_SIZE (1UL << MALLOC_SHIFT_MAX) +/* + * ZERO_SIZE_PTR will be returned for zero sized kmalloc requests. + * + * Dereferencing ZERO_SIZE_PTR will lead to a distinct access fault. + * + * ZERO_SIZE_PTR can be passed to free though in the same way that NULL can. + * Both make free a no-op. + */ +#define ZERO_SIZE_PTR ((void *)16) + +#define ZERO_OR_NULL_PTR(x) ((unsigned long)(x) <= \ + (unsigned long)ZERO_SIZE_PTR) + #if IN_PROPER void *malloc(size_t) __alloc_size(1); size_t malloc_usable_size(void *); diff --git a/test/self/malloc.c b/test/self/malloc.c index 79a32b5d97c7..52f9fc344c1a 100644 --- a/test/self/malloc.c +++ b/test/self/malloc.c @@ -122,7 +122,8 @@ static void test_malloc(void) p = expect_alloc_ok(malloc(0)); tmp = expect_alloc_ok(malloc(0)); - __expect_cond(p != tmp, true, "allocate distinct 0-size buffers", __func__, __LINE__); + __expect_cond(p == tmp, true, "allocate equal 0-size buffers", __func__, __LINE__); + __expect_cond(p == ZERO_SIZE_PTR, true, "get ZERO_SIZE_PTR for 0-size buffers", __func__, __LINE__); free(p); free(tmp); -- 2.39.5
