Deprecated Interfaces, Language Features, Attributes, and Conventions

In a perfect world, it would be possible to convert all instances of some deprecated API into the new API and entirely remove the old API in a single development cycle. However, due to the size of the kernel, the maintainership hierarchy, and timing, it's not always feasible to do these kinds of conversions at once. This means that new instances may sneak into the kernel while old ones are being removed, only making the amount of work to remove the API grow. In order to educate developers about what has been deprecated and why, this list has been created as a place to point when uses of deprecated things are proposed for inclusion in the kernel.

__deprecated

While this attribute does visually mark an interface as deprecated, it does not produce warnings during builds any more because one of the standing goals of the kernel is to build without warnings and no one was actually doing anything to remove these deprecated interfaces. While using __deprecated is nice to note an old API in a header file, it isn't the full solution. Such interfaces must either be fully removed from the kernel, or added to this file to discourage others from using them in the future.

open-coded arithmetic in allocator arguments

Dynamic size calculations (especially multiplication) should not be performed in memory allocator (or similar) function arguments due to the 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.)

For example, do not use count * size as an argument, as in:

foo = kmalloc(count * size, GFP_KERNEL);

Instead, the 2-factor form of the allocator should be used:

foo = kmalloc_array(count, size, GFP_KERNEL);

If no 2-factor form is available, the saturate-on-overflow helpers should be used:

bar = vmalloc(array_size(count, size));

Another common case to avoid is calculating the size of a structure with a trailing array of others structures, as in:

header = kzalloc(sizeof(*header) + count * sizeof(*header->item),
                 GFP_KERNEL);

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.

simple_strtol(), simple_strtoll(), simple_strtoul(), simple_strtoull()

The simple_strtol(), simple_strtoll(), simple_strtoul(), and simple_strtoull() functions explicitly ignore overflows, which may lead to unexpected results in callers. The respective kstrtol(), kstrtoll(), kstrtoul(), and kstrtoull() functions tend to be the correct replacements, though note that those require the string to be 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().

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 will need an explicit memset() added.)

If a caller is using non-NUL-terminated strings, strncpy() can still be used, but destinations should be marked with the __nonstring attribute to avoid future compiler warnings.

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().

Variable Length Arrays (VLAs)

Using stack VLAs produces much worse machine code than statically sized stack arrays. While these non-trivial performance issues are reason enough to eliminate VLAs, they are also a security risk. Dynamic growth of a stack array may exceed the remaining memory in the stack segment. This could lead to a crash, possible overwriting sensitive contents at the end of the stack (when built without CONFIG_THREAD_INFO_IN_TASK=y), or overwriting memory adjacent to the stack (when built without CONFIG_VMAP_STACK=y)