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.
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);
simple_strtol(), simple_strtoll(), simple_strtoul(), simple_strtoull()¶
explicitly ignore overflows, which may lead to unexpected results
in callers. The respective
kstrtoull() functions tend to be the
correct replacements, though note that those require the string to be
NUL or newline terminated.
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
strncpy() on NUL-terminated strings¶
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() still needing NUL-padding will need an
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
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)
Implicit switch case fall-through¶
The C language allows switch cases to “fall-through” when a “break” statement is missing at the end of a case. This, however, introduces ambiguity in the code, as it’s not always clear if the missing break is intentional or a bug.
As there have been a long list of flaws due to missing “break” statements, we no longer allow “implicit fall-through”.
In order to identify intentional fall-through cases, we have adopted a pseudo-keyword macro ‘fallthrough’ which expands to gcc’s extension __attribute__((__fallthrough__)). Statement Attributes
When the C17/C18 [[fallthrough]] syntax is more commonly supported by C compilers, static analyzers, and IDEs, we can switch to using that syntax for the macro pseudo-keyword.
All switch/case blocks must end in one of:
break; fallthrough; continue; goto <label>; return [expression];