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    SubjectRe: [PATCH] fs: make d_path-like functions all have unsigned size
    On Tue, Jul 27, 2021 at 02:50:19PM +0000, Al Viro wrote:
    > On Tue, Jul 27, 2021 at 12:36:25PM +0200, Greg Kroah-Hartman wrote:
    > > When running static analysis tools to find where signed values could
    > > potentially wrap the family of d_path() functions turn out to trigger a
    > > lot of mess. In evaluating the code, all of these usages seem safe, but
    > > pointer math is involved so if a negative number is ever somehow passed
    > > into these functions, memory can be traversed backwards in ways not
    > > intended.
    > >
    > > Resolve all of the abuguity by just making "size" an unsigned value,
    > > which takes the guesswork out of everything involved.
    >
    > TBH, I'm not sure it's the right approach. Huge argument passed to d_path()
    > is a bad idea, no matter what. Do you really want to have the damn thing
    > try and fill 3Gb of buffer, all while holding rcu_read_lock() and a global
    > spinlock or two? Hell, s/3Gb/1Gb/ and it won't get any better...
    >
    >
    > How about we do this instead:
    >
    > d_path(const struct path *path, char *buf, int buflen)
    > {
    > if (unlikely((unsigned)buflen > 0x8000)) {
    > buf += (unsigned)buflen - 0x8000;
    > buflen = 0x8000;
    > }
    > as in mainline
    > }
    >
    > and take care of both issues?

    umm ... what if someone passes in -ENOMEM as buflen? Not saying we
    have such a path right now, but I could imagine it happening.

    if (unlikely(buflen < 0))
    return ERR_PTR(buflen);
    if (unlikely(buflen > 0x8000)) {
    buf += buflen - 0x8000;
    buflen = 0x8000;
    }
    ...

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