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    SubjectRe: UTF-8 practically vs. theoretically in the VFS API (was: Re: JFS default behavior)
    On Tue, Feb 17, 2004 at 07:29:18PM +0000, Jamie Lokier wrote:
    > What happens is that one program or library checks an incoming path
    > for ".." components - that code knows nothing about UTF-8 of course.
    > Then it passes the string to another program which assumes the path
    > has been subject to appropriate security checks, munges it in UTF-8,
    > and eventually does a file operation with it. The munging generates
    > ".." components from non-minimal UTF-8 forms - if it's not obeying the
    > Unicode rejection requirement (which wasn't in earlier versions), that is.

    Why the hell would it _ever_ do such normalization?

    > A realistic example is where the second program reads files whose
    > paths are mentioned in a text file which is parsed as UTF-8, after the
    > first program has done a security check by grepping for ".."
    > components.
    > Unicode says the second program shouldn't accept malformed UTF-8,
    > precisely because in real scenarios (like this one) there's a mix of
    > programs and libraries, some aware of UTF-8, some not, and the latter
    > are involved in security decisions.
    > Here on linux-kernel we're saying that if the second program accepts
    > any old byte sequence in a filename, it should preserve the byte
    > sequence exactly. But any program whose parser-tokeniser is scanning
    > UTF-8 is very unlikely to do that - it's just too complicated to say
    > some bits of a text stream must be remembered as literal bytes, and
    > others must be scanned as multibyte characters.

    So what you are saying is that conversion of invalid multibyte sequences
    into non-error wide chars followed by conversion back into UTF-8 can
    lead to trouble? *DUH*

    > The holes only arise because software which is interpreting UTF-8 is
    > mixed with software which isn't. That's one of the most useful
    > features of UTF-8, after all - that's why we use it for filenames.

    The holes only arise because software which is interpreting UTF-8 doesn't
    care to do it properly. Software that doesn't interpret it (including the
    kernel) doesn't enter the picture at all.
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