Adding upstream version 6.6.15.upstream/6.6.15

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 374 insertions, 0 deletions

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+.. SPDX-License-Identifier: GPL-2.0
+
+.. _deprecated:
+
+=====================================================================
+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
+<https://git.kernel.org/linus/771c035372a036f83353eef46dbb829780330234>`_
+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.
+
+BUG() and BUG_ON()
+------------------
+Use WARN() and WARN_ON() instead, and handle the "impossible"
+error condition as gracefully as possible. While the BUG()-family
+of APIs were originally designed to act as an "impossible situation"
+assert and to kill a kernel thread "safely", they turn out to just be
+too risky. (e.g. "In what order do locks need to be released? Have
+various states been restored?") Very commonly, using BUG() will
+destabilize a system or entirely break it, which makes it impossible
+to debug or even get viable crash reports. Linus has `very strong
+<https://lore.kernel.org/lkml/CA+55aFy6jNLsywVYdGp83AMrXBo_P-pkjkphPGrO=82SPKCpLQ@mail.gmail.com/>`_
+feelings `about this
+<https://lore.kernel.org/lkml/CAHk-=whDHsbK3HTOpTF=ue_o04onRwTEaK_ZoJp_fjbqq4+=Jw@mail.gmail.com/>`_.
+
+Note that the WARN()-family should only be used for "expected to
+be unreachable" situations. If you want to warn about "reachable
+but undesirable" situations, please use the pr_warn()-family of
+functions. System owners may have set the *panic_on_warn* sysctl,
+to make sure their systems do not continue running in the face of
+"unreachable" conditions. (For example, see commits like `this one
+<https://git.kernel.org/linus/d4689846881d160a4d12a514e991a740bcb5d65a>`_.)
+
+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. 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::
+
+	foo = kmalloc(count * size, GFP_KERNEL);
+
+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 = 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::
+
+	header = kzalloc(sizeof(*header) + count * sizeof(*header->item),
+			 GFP_KERNEL);
+
+Instead, use the helper::
+
+	header = kzalloc(struct_size(header, item, count), GFP_KERNEL);
+
+.. 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()
+----------------------------------------------------------------------
+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(), 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.
+
+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. For cases still needing
+NUL-padding, strtomem_pad() can be used.
+
+strlcpy()
+---------
+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
+-------------------
+Traditionally, using "%p" in format strings would lead to regular address
+exposure flaws in dmesg, proc, sysfs, etc. Instead of leaving these to
+be exploitable, all "%p" uses in the kernel are being printed as a hashed
+value, rendering them unusable for addressing. New uses of "%p" should not
+be added to the kernel. For text addresses, using "%pS" is likely better,
+as it produces the more useful symbol name instead. For nearly everything
+else, just do not add "%p" at all.
+
+Paraphrasing Linus's current `guidance <https://lore.kernel.org/lkml/CA+55aFwQEd_d40g4mUCSsVRZzrFPUJt74vc6PPpb675hYNXcKw@mail.gmail.com/>`_:
+
+- If the hashed "%p" value is pointless, ask yourself whether the pointer
+  itself is important. Maybe it should be removed entirely?
+- If you really think the true pointer value is important, why is some
+  system state or user privilege level considered "special"? If you think
+  you can justify it (in comments and commit log) well enough to stand
+  up to Linus's scrutiny, maybe you can use "%px", along with making sure
+  you have sensible permissions.
+
+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)
+-----------------------------
+Using stack VLAs produces much worse machine code than statically
+sized stack arrays. While these non-trivial `performance issues
+<https://git.kernel.org/linus/02361bc77888>`_ 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 to the next case
+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. For example, it's not obvious just from
+looking at the code if `STATE_ONE` is intentionally designed to fall
+through into `STATE_TWO`::
+
+	switch (value) {
+	case STATE_ONE:
+		do_something();
+	case STATE_TWO:
+		do_other();
+		break;
+	default:
+		WARN("unknown state");
+	}
+
+As there have been a long list of flaws `due to missing "break" statements
+<https://cwe.mitre.org/data/definitions/484.html>`_, 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__))
+<https://gcc.gnu.org/onlinedocs/gcc/Statement-Attributes.html>`_.
+(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];
+
+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);
+		};
+	};