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    SubjectRE: [PATCH RFCv4 1/4] fs: introduce helper d_path_unsafe()
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    Hi Andy
    Thanks for the comments, I will resolve you mentioned typo/grammar issues

    Some answer below

    > -----Original Message-----
    > From: Andy Shevchenko <andy.shevchenko@gmail.com>
    > Sent: Wednesday, June 16, 2021 4:41 AM
    > To: Justin He <Justin.He@arm.com>
    > Cc: Petr Mladek <pmladek@suse.com>; Steven Rostedt <rostedt@goodmis.org>;
    > Sergey Senozhatsky <senozhatsky@chromium.org>; Andy Shevchenko
    > <andriy.shevchenko@linux.intel.com>; Rasmus Villemoes
    > <linux@rasmusvillemoes.dk>; Jonathan Corbet <corbet@lwn.net>; Alexander
    > Viro <viro@zeniv.linux.org.uk>; Linus Torvalds <torvalds@linux-
    > foundation.org>; Peter Zijlstra (Intel) <peterz@infradead.org>; Eric
    > Biggers <ebiggers@google.com>; Ahmed S. Darwish <a.darwish@linutronix.de>;
    > Linux Documentation List <linux-doc@vger.kernel.org>; Linux Kernel Mailing
    > List <linux-kernel@vger.kernel.org>; Linux FS Devel <linux-
    > fsdevel@vger.kernel.org>; Matthew Wilcox <willy@infradead.org>
    > Subject: Re: [PATCH RFCv4 1/4] fs: introduce helper d_path_unsafe()
    >
    > On Tue, Jun 15, 2021 at 6:56 PM Jia He <justin.he@arm.com> wrote:
    > >
    > > This helper is similar to d_path except that it doesn't take any
    > > seqlock/spinlock. It is typical for debugging purpose. Besides,
    >
    > purposes
    >
    > > an additional return value *prenpend_len* is used to get the full
    > > path length of the dentry.
    > >
    > > prepend_name_with_len() enhances the behavior of prepend_name().
    > > Previously it will skip the loop at once in __prepen_path() when the
    > > space is not enough. __prepend_path() gets the full length of dentry
    > > together with the parent recusively.
    >
    > recursively
    >
    > >
    > > Besides, if someone invokes snprintf with small but positive space,
    > > prepend_name_with() needs to move and copy the string partially.
    > >
    > > More than that, kasnprintf will pass NULL _buf_ and _end_, hence
    >
    > kasprintf()
    >
    > > it returns at the very beginning with false in this case;
    > >
    > > Suggested-by: Matthew Wilcox <willy@infradead.org>
    > > Signed-off-by: Jia He <justin.he@arm.com>
    > > ---
    > > fs/d_path.c | 83 +++++++++++++++++++++++++++++++++++++++++-
    > > include/linux/dcache.h | 1 +
    > > 2 files changed, 82 insertions(+), 2 deletions(-)
    > >
    > > diff --git a/fs/d_path.c b/fs/d_path.c
    > > index 23a53f7b5c71..4fc224eadf58 100644
    > > --- a/fs/d_path.c
    > > +++ b/fs/d_path.c
    > > @@ -68,9 +68,66 @@ static bool prepend_name(struct prepend_buffer *p,
    > const struct qstr *name)
    > > return true;
    > > }
    > >
    > > +static bool prepend_name_with_len(struct prepend_buffer *p, const struct
    > qstr *name,
    > > + int orig_buflen)
    > > +{
    > > + const char *dname = smp_load_acquire(&name->name); /* ^^^ */
    >
    > What does this funny comment mean?

    It is inherited from the prepend_name()

    >
    > > + int dlen = READ_ONCE(name->len);
    > > + char *s;
    > > + int last_len = p->len;
    > > +
    > > + p->len -= dlen + 1;
    > > +
    > > + if (unlikely(!p->buf))
    > > + return false;
    > > +
    > > + if (orig_buflen <= 0)
    > > + return false;
    > > + /*
    > > + * The first time we overflow the buffer. Then fill the string
    > > + * partially from the beginning
    > > + */
    > > + if (unlikely(p->len < 0)) {
    > > + int buflen = strlen(p->buf);
    > > +
    > > + s = p->buf;
    > > +
    > > + /* Still have small space to fill partially */
    > > + if (last_len > 0) {
    > > + p->buf -= last_len;
    > > + buflen += last_len;
    > > + }
    > > +
    > > + if (buflen > dlen + 1) {
    > > + /* This dentry name can be fully filled */
    > > + memmove(p->buf + dlen + 1, s, buflen - dlen - 1);
    > > + p->buf[0] = '/';
    > > + memcpy(p->buf + 1, dname, dlen);
    > > + } else if (buflen > 0) {
    > > + /* Partially filled, and drop last dentry name */
    > > + p->buf[0] = '/';
    > > + memcpy(p->buf + 1, dname, buflen - 1);
    > > + }
    > > +
    > > + return false;
    > > + }
    > > +
    > > + s = p->buf -= dlen + 1;
    > > + *s++ = '/';
    >
    > > + while (dlen--) {
    > > + char c = *dname++;
    > > +
    > > + if (!c)
    > > + break;
    > > + *s++ = c;
    >
    > I'm wondering why can't memcpy() be used here as well.


    From the comments of commit 7a5cf791:

    >However, there may be mismatch between length and pointer.
    > * The length cannot be trusted, we need to copy it byte-by-byte until
    > * the length is reached or a null byte is found.

    Seems we shouldn't use memcpy/strcpy here

    >
    > > + }
    > > + return true;
    > > +}
    > > static int __prepend_path(const struct dentry *dentry, const struct
    > mount *mnt,
    > > const struct path *root, struct prepend_buffer
    > *p)
    > > {
    > > + int orig_buflen = p->len;
    > > +
    > > while (dentry != root->dentry || &mnt->mnt != root->mnt) {
    > > const struct dentry *parent = READ_ONCE(dentry->d_parent);
    > >
    > > @@ -97,8 +154,7 @@ static int __prepend_path(const struct dentry *dentry,
    > const struct mount *mnt,
    > > return 3;
    > >
    > > prefetch(parent);
    > > - if (!prepend_name(p, &dentry->d_name))
    > > - break;
    > > + prepend_name_with_len(p, &dentry->d_name, orig_buflen);
    > > dentry = parent;
    > > }
    > > return 0;
    > > @@ -263,6 +319,29 @@ char *d_path(const struct path *path, char *buf, int
    > buflen)
    > > }
    > > EXPORT_SYMBOL(d_path);
    > >
    > > +/**
    > > + * d_path_unsafe - fast return the full path of a dentry without taking
    > > + * any seqlock/spinlock. This helper is typical for debugging purpose.
    >
    > purposes
    >
    > Haven't you got kernel doc validation warnings? Please, describe
    > parameters as well.

    I will check it and update if there is a warning, thanks for the reminder.


    --
    Cheers,
    Justin (Jia He)



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