Once in a while %p usage comes up, and I've needed to have a reference to point people to. Add %p details to deprecated.rst. Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx> --- v2: rewrite much of the text to be more clear (James Troup) --- Documentation/process/deprecated.rst | 22 ++++++++++++++++++++++ 1 file changed, 22 insertions(+) diff --git a/Documentation/process/deprecated.rst b/Documentation/process/deprecated.rst index f9f196d3a69b..8965446f0b71 100644 --- a/Documentation/process/deprecated.rst +++ b/Documentation/process/deprecated.rst @@ -109,6 +109,28 @@ 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 :c:func:`strscpy`. +%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@xxxxxxxxxxxxxx/>`_: + +- 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. + +And finally, know that a toggle for "%p" hashing will `not be accepted <https://lore.kernel.org/lkml/CA+55aFwieC1-nAs+NFq9RTwaR8ef9hWa4MjNBWL41F-8wM49eA@xxxxxxxxxxxxxx/>`_. + Variable Length Arrays (VLAs) ----------------------------- Using stack VLAs produces much worse machine code than statically -- 2.20.1 -- Kees Cook
