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docs: deprecated.rst: Expand str*cpy() replacement notes 

The notes on replacing the deprecated str*cpy() functions didn't call
enough attention to the change in return type. Add these details and
clean up the language a bit more.

Signed-off-by: Kees Cook <keescook@chromium.org>
Acked-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/20201015231730.2138505-1-keescook@chromium.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
This commit is contained in:
Kees Cook 2020-10-15 16:17:31 -07:00 committed by Jonathan Corbet
parent bb7a2c6362
commit 27def953b6
1 changed files with 26 additions and 18 deletions
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44
Documentation/process/deprecated.rst
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@ -106,23 +106,29 @@ 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().)
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(), 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, strncpy() can
still be used, but destinations should be marked with the `__nonstring
@ -131,10 +137,12 @@ 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().
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
-------------------