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    SubjectRE: [PATCH RFCv2 2/3] lib/vsprintf.c: make %pD print full path for file
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    Hi Matthew

    > -----Original Message-----
    > From: Matthew Wilcox <willy@infradead.org>
    > Sent: Friday, May 28, 2021 11:22 PM
    > To: Justin He <Justin.He@arm.com>
    > Cc: Linus Torvalds <torvalds@linux-foundation.org>; 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>; Luca Coelho <luciano.coelho@intel.com>;
    > Kalle Valo <kvalo@codeaurora.org>; David S. Miller <davem@davemloft.net>;
    > Jakub Kicinski <kuba@kernel.org>; Heiko Carstens <hca@linux.ibm.com>;
    > Vasily Gorbik <gor@linux.ibm.com>; Christian Borntraeger
    > <borntraeger@de.ibm.com>; Johannes Berg <johannes.berg@intel.com>; linux-
    > doc@vger.kernel.org; linux-kernel@vger.kernel.org; linux-
    > wireless@vger.kernel.org; netdev@vger.kernel.org; linux-
    > s390@vger.kernel.org
    > Subject: Re: [PATCH RFCv2 2/3] lib/vsprintf.c: make %pD print full path
    > for file
    >
    > On Fri, May 28, 2021 at 03:09:28PM +0000, Justin He wrote:
    > > > I'm not sure why it's so complicated. p->len records how many bytes
    > > > are needed for the entire path; can't you just return -p->len ?
    > >
    > > prepend_name() will return at the beginning if p->len is <0 in this case,
    > > we can't even get the correct full path size if keep __prepend_path
    > unchanged.
    > > We need another new helper __prepend_path_size() to get the full path
    > size
    > > regardless of the negative value p->len.
    >
    > It's a little hard to follow, based on just the patches. Is there a
    > git tree somewhere of Al's patches that you're based on?
    >
    > Seems to me that prepend_name() is just fine because it updates p->len
    > before returning false:
    >
    > static bool prepend_name(struct prepend_buffer *p, const struct qstr
    > *name)
    > {
    > const char *dname = smp_load_acquire(&name->name); /* ^^^ */
    > u32 dlen = READ_ONCE(name->len);
    > char *s;
    >
    > p->len -= dlen + 1;
    > if (unlikely(p->len < 0))
    > return false;
    >
    > I think the only change you'd need to make for vsnprintf() is in
    > prepend_path():
    >
    > - if (!prepend_name(&b, &dentry->d_name))
    > - break;
    > + prepend_name(&b, &dentry->d_name);
    >
    > Would that hurt anything else?
    >

    It almost works except the snprintf case,
    Consider,assuming filp path is 256 bytes, 2 dentries "/root/$long_string":
    snprintf(buffer, 128, "%pD", filp);
    p->len is positive at first, but negative after prepend_name loop.
    So, it will not fill any bytes in _buffer_.
    But in theory, it should fill the beginning 127 bytes and '\0'.

    What do you think of it?

    --
    Cheers,
    Justin (Jia He)


    > > More than that, even the 1st vsnprintf could have _end_ > _buf_ in some
    > case:
    > > What if printk("%pD", filp) ? The 1st vsnprintf has positive (end-buf).
    >
    > I don't understand the problem ... if p->len is positive, then you
    > succeeded. if p->len is negative then -p->len is the expected return
    > value from vsnprintf(). No?

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