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Diffstat (limited to 'Documentation/process/deprecated.rst')
| -rw-r--r-- | Documentation/process/deprecated.rst | 230 |
1 files changed, 201 insertions, 29 deletions
diff --git a/Documentation/process/deprecated.rst b/Documentation/process/deprecated.rst index 652e2aa02a66..1f7f3e6c9cda 100644 --- a/Documentation/process/deprecated.rst +++ b/Documentation/process/deprecated.rst @@ -59,8 +59,9 @@ risk of them overflowing. This could lead to values wrapping around and a smaller allocation being made than the caller was expecting. Using those allocations could lead to linear overflows of heap memory and other misbehaviors. (One exception to this is literal values where the compiler -can warn if they might overflow. Though using literals for arguments as -suggested below is also harmless.) +can warn if they might overflow. However, the preferred way in these +cases is to refactor the code as suggested below to avoid the open-coded +arithmetic.) For example, do not use ``count * size`` as an argument, as in:: @@ -70,10 +71,13 @@ Instead, the 2-factor form of the allocator should be used:: foo = kmalloc_array(count, size, GFP_KERNEL); +Specifically, kmalloc() can be replaced with kmalloc_array(), and +kzalloc() can be replaced with kcalloc(). + If no 2-factor form is available, the saturate-on-overflow helpers should be used:: - bar = vmalloc(array_size(count, size)); + bar = dma_alloc_coherent(dev, array_size(count, size), &dma, GFP_KERNEL); Another common case to avoid is calculating the size of a structure with a trailing array of others structures, as in:: @@ -85,9 +89,25 @@ Instead, use the helper:: header = kzalloc(struct_size(header, item, count), GFP_KERNEL); -See array_size(), array3_size(), and struct_size(), -for more details as well as the related check_add_overflow() and -check_mul_overflow() family of functions. +.. note:: If you are using struct_size() on a structure containing a zero-length + or a one-element array as a trailing array member, please refactor such + array usage and switch to a `flexible array member + <#zero-length-and-one-element-arrays>`_ instead. + +For other calculations, please compose the use of the size_mul(), +size_add(), and size_sub() helpers. For example, in the case of:: + + foo = krealloc(current_size + chunk_size * (count - 3), GFP_KERNEL); + +Instead, use the helpers:: + + foo = krealloc(size_add(current_size, + size_mul(chunk_size, + size_sub(count, 3))), GFP_KERNEL); + +For more details, also see array3_size() and flex_array_size(), +as well as the related check_mul_overflow(), check_add_overflow(), +check_sub_overflow(), and check_shl_overflow() family of functions. simple_strtol(), simple_strtoll(), simple_strtoul(), simple_strtoull() ---------------------------------------------------------------------- @@ -101,35 +121,46 @@ NUL or newline terminated. strcpy() -------- -strcpy() performs no bounds checking on the destination -buffer. This could result in linear overflows beyond the -end of the buffer, leading to all kinds of misbehaviors. While -`CONFIG_FORTIFY_SOURCE=y` and various compiler flags help reduce the -risk of using this function, there is no good reason to add new uses of -this function. The safe replacement is strscpy(). +strcpy() performs no bounds checking on the destination buffer. This +could result in linear overflows beyond the end of the buffer, leading to +all kinds of misbehaviors. While `CONFIG_FORTIFY_SOURCE=y` and various +compiler flags help reduce the risk of using this function, there is +no good reason to add new uses of this function. The safe replacement +is strscpy(), though care must be given to any cases where the return +value of strcpy() was used, since strscpy() does not return a pointer to +the destination, but rather a count of non-NUL bytes copied (or negative +errno when it truncates). strncpy() on NUL-terminated strings ----------------------------------- -Use of strncpy() does not guarantee that the destination buffer -will be NUL terminated. This can lead to various linear read overflows -and other misbehavior due to the missing termination. It also NUL-pads the -destination buffer if the source contents are shorter than the destination -buffer size, which may be a needless performance penalty for callers using -only NUL-terminated strings. The safe replacement is strscpy(). -(Users of strscpy() still needing NUL-padding should instead -use strscpy_pad().) - -If a caller is using non-NUL-terminated strings, strncpy()() can -still be used, but destinations should be marked with the `__nonstring +Use of strncpy() does not guarantee that the destination buffer will +be NUL terminated. This can lead to various linear read overflows and +other misbehavior due to the missing termination. It also NUL-pads +the destination buffer if the source contents are shorter than the +destination buffer size, which may be a needless performance penalty +for callers using only NUL-terminated strings. + +When the destination is required to be NUL-terminated, the replacement is +strscpy(), though care must be given to any cases where the return value +of strncpy() was used, since strscpy() does not return a pointer to the +destination, but rather a count of non-NUL bytes copied (or negative +errno when it truncates). Any cases still needing NUL-padding should +instead use strscpy_pad(). + +If a caller is using non-NUL-terminated strings, strtomem() should be +used, and the destinations should be marked with the `__nonstring <https://gcc.gnu.org/onlinedocs/gcc/Common-Variable-Attributes.html>`_ -attribute to avoid future compiler warnings. +attribute to avoid future compiler warnings. For cases still needing +NUL-padding, strtomem_pad() can be used. strlcpy() --------- -strlcpy() reads the entire source buffer first, possibly exceeding -the given limit of bytes to copy. This is inefficient and can lead to -linear read overflows if a source string is not NUL-terminated. The -safe replacement is strscpy(). +strlcpy() reads the entire source buffer first (since the return value +is meant to match that of strlen()). This read may exceed the destination +size limit. This is both inefficient and can lead to linear read overflows +if a source string is not NUL-terminated. The safe replacement is strscpy(), +though care must be given to any cases where the return value of strlcpy() +is used, since strscpy() will return negative errno values when it truncates. %p format specifier ------------------- @@ -151,7 +182,9 @@ Paraphrasing Linus's current `guidance <https://lore.kernel.org/lkml/CA+55aFwQEd up to Linus's scrutiny, maybe you can use "%px", along with making sure you have sensible permissions. -And finally, know that a toggle for "%p" hashing will `not be accepted <https://lore.kernel.org/lkml/CA+55aFwieC1-nAs+NFq9RTwaR8ef9hWa4MjNBWL41F-8wM49eA@mail.gmail.com/>`_. +If you are debugging something where "%p" hashing is causing problems, +you can temporarily boot with the debug flag "`no_hash_pointers +<https://git.kernel.org/linus/5ead723a20e0447bc7db33dc3070b420e5f80aa6>`_". Variable Length Arrays (VLAs) ----------------------------- @@ -200,3 +233,142 @@ All switch/case blocks must end in one of: * continue; * goto <label>; * return [expression]; + +Zero-length and one-element arrays +---------------------------------- +There is a regular need in the kernel to provide a way to declare having +a dynamically sized set of trailing elements in a structure. Kernel code +should always use `"flexible array members" <https://en.wikipedia.org/wiki/Flexible_array_member>`_ +for these cases. The older style of one-element or zero-length arrays should +no longer be used. + +In older C code, dynamically sized trailing elements were done by specifying +a one-element array at the end of a structure:: + + struct something { + size_t count; + struct foo items[1]; + }; + +This led to fragile size calculations via sizeof() (which would need to +remove the size of the single trailing element to get a correct size of +the "header"). A `GNU C extension <https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html>`_ +was introduced to allow for zero-length arrays, to avoid these kinds of +size problems:: + + struct something { + size_t count; + struct foo items[0]; + }; + +But this led to other problems, and didn't solve some problems shared by +both styles, like not being able to detect when such an array is accidentally +being used _not_ at the end of a structure (which could happen directly, or +when such a struct was in unions, structs of structs, etc). + +C99 introduced "flexible array members", which lacks a numeric size for +the array declaration entirely:: + + struct something { + size_t count; + struct foo items[]; + }; + +This is the way the kernel expects dynamically sized trailing elements +to be declared. It allows the compiler to generate errors when the +flexible array does not occur last in the structure, which helps to prevent +some kind of `undefined behavior +<https://git.kernel.org/linus/76497732932f15e7323dc805e8ea8dc11bb587cf>`_ +bugs from being inadvertently introduced to the codebase. It also allows +the compiler to correctly analyze array sizes (via sizeof(), +`CONFIG_FORTIFY_SOURCE`, and `CONFIG_UBSAN_BOUNDS`). For instance, +there is no mechanism that warns us that the following application of the +sizeof() operator to a zero-length array always results in zero:: + + struct something { + size_t count; + struct foo items[0]; + }; + + struct something *instance; + + instance = kmalloc(struct_size(instance, items, count), GFP_KERNEL); + instance->count = count; + + size = sizeof(instance->items) * instance->count; + memcpy(instance->items, source, size); + +At the last line of code above, ``size`` turns out to be ``zero``, when one might +have thought it represents the total size in bytes of the dynamic memory recently +allocated for the trailing array ``items``. Here are a couple examples of this +issue: `link 1 +<https://git.kernel.org/linus/f2cd32a443da694ac4e28fbf4ac6f9d5cc63a539>`_, +`link 2 +<https://git.kernel.org/linus/ab91c2a89f86be2898cee208d492816ec238b2cf>`_. +Instead, `flexible array members have incomplete type, and so the sizeof() +operator may not be applied <https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html>`_, +so any misuse of such operators will be immediately noticed at build time. + +With respect to one-element arrays, one has to be acutely aware that `such arrays +occupy at least as much space as a single object of the type +<https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html>`_, +hence they contribute to the size of the enclosing structure. This is prone +to error every time people want to calculate the total size of dynamic memory +to allocate for a structure containing an array of this kind as a member:: + + struct something { + size_t count; + struct foo items[1]; + }; + + struct something *instance; + + instance = kmalloc(struct_size(instance, items, count - 1), GFP_KERNEL); + instance->count = count; + + size = sizeof(instance->items) * instance->count; + memcpy(instance->items, source, size); + +In the example above, we had to remember to calculate ``count - 1`` when using +the struct_size() helper, otherwise we would have --unintentionally-- allocated +memory for one too many ``items`` objects. The cleanest and least error-prone way +to implement this is through the use of a `flexible array member`, together with +struct_size() and flex_array_size() helpers:: + + struct something { + size_t count; + struct foo items[]; + }; + + struct something *instance; + + instance = kmalloc(struct_size(instance, items, count), GFP_KERNEL); + instance->count = count; + + memcpy(instance->items, source, flex_array_size(instance, items, instance->count)); + +There are two special cases of replacement where the DECLARE_FLEX_ARRAY() +helper needs to be used. (Note that it is named __DECLARE_FLEX_ARRAY() for +use in UAPI headers.) Those cases are when the flexible array is either +alone in a struct or is part of a union. These are disallowed by the C99 +specification, but for no technical reason (as can be seen by both the +existing use of such arrays in those places and the work-around that +DECLARE_FLEX_ARRAY() uses). For example, to convert this:: + + struct something { + ... + union { + struct type1 one[0]; + struct type2 two[0]; + }; + }; + +The helper must be used:: + + struct something { + ... + union { + DECLARE_FLEX_ARRAY(struct type1, one); + DECLARE_FLEX_ARRAY(struct type2, two); + }; + }; |