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    SubjectRe: [PATCH 1/2] string: Add stracpy and stracpy_pad mechanisms
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    On 23/07/2019 02.38, Joe Perches wrote:
    > Several uses of strlcpy and strscpy have had defects because the
    > last argument of each function is misused or typoed.
    >
    > Add macro mechanisms to avoid this defect.
    >
    > stracpy (copy a string to a string array) must have a string
    > array as the first argument (to) and uses sizeof(to) as the
    > size.
    >
    > These mechanisms verify that the to argument is an array of
    > char or other compatible types

    yes

    like u8 or unsigned char.

    no. "unsigned char" aka u8, "signed char" aka s8 and plain char are not
    __builtin_types_compatible_p to one another.

    > A BUILD_BUG is emitted when the type of to is not compatible.
    >
    > Signed-off-by: Joe Perches <joe@perches.com>
    > ---
    > include/linux/string.h | 41 +++++++++++++++++++++++++++++++++++++++++
    > 1 file changed, 41 insertions(+)
    >
    > diff --git a/include/linux/string.h b/include/linux/string.h
    > index 4deb11f7976b..f80b0973f0e5 100644
    > --- a/include/linux/string.h
    > +++ b/include/linux/string.h
    > @@ -35,6 +35,47 @@ ssize_t strscpy(char *, const char *, size_t);
    > /* Wraps calls to strscpy()/memset(), no arch specific code required */
    > ssize_t strscpy_pad(char *dest, const char *src, size_t count);
    >
    > +/**
    > + * stracpy - Copy a C-string into an array of char
    > + * @to: Where to copy the string, must be an array of char and not a pointer
    > + * @from: String to copy, may be a pointer or const char array
    > + *
    > + * Helper for strscpy.
    > + * Copies a maximum of sizeof(@to) bytes of @from with %NUL termination.
    > + *
    > + * Returns:
    > + * * The number of characters copied (not including the trailing %NUL)
    > + * * -E2BIG if @to is a zero size array.

    Well, yes, but more importantly and generally: -E2BIG if the copy
    including %NUL didn't fit. [The zero size array thing could be made into
    a build bug for these stra* variants if one thinks that might actually
    occur in real code.]

    > + */
    > +#define stracpy(to, from) \
    > +({ \
    > + size_t size = ARRAY_SIZE(to); \
    > + BUILD_BUG_ON(!__same_type(typeof(*to), char)); \
    > + \

    ARRAY_SIZE should ensure to is indeed an array, but it doesn't hurt to
    spell the second condition

    BUILD_BUG_ON(!__same_type(typeof(to), char[]))

    (the gcc docs explicitly mention that "The type 'int[]' and 'int[5]' are
    compatible.) - just in case that line gets copy-pasted somewhere that
    doesn't have another must-be-array check nearby.

    You should use a more "unique" identifier than "size". from could be
    some expression that refers to such a variable from the surrounding scope.

    Rasmus

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