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

[Justin He]

Hi Matthew

> -----Original Message-----
> From: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> Sent: Friday, May 28, 2021 10:53 PM
> To: Justin He <Justin.He@xxxxxxx>
> Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>; Petr Mladek
> <pmladek@xxxxxxxx>; Steven Rostedt <rostedt@xxxxxxxxxxx>; Sergey
> Senozhatsky <senozhatsky@xxxxxxxxxxxx>; Andy Shevchenko
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> Viro <viro@xxxxxxxxxxxxxxxxxx>; Luca Coelho <luciano.coelho@xxxxxxxxx>;
> Kalle Valo <kvalo@xxxxxxxxxxxxxx>; David S. Miller <davem@xxxxxxxxxxxxx>;
> Jakub Kicinski <kuba@xxxxxxxxxx>; Heiko Carstens <hca@xxxxxxxxxxxxx>;
> Vasily Gorbik <gor@xxxxxxxxxxxxx>; Christian Borntraeger
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> Subject: Re: [PATCH RFCv2 2/3] lib/vsprintf.c: make %pD print full path
> for file
>
> On Fri, May 28, 2021 at 02:22:01PM +0000, Justin He wrote:
> > > On Fri, May 28, 2021 at 07:39:50PM +0800, Jia He wrote:
> > > > We have '%pD' for printing a filename. It may not be perfect (by
> > > > default it only prints one component.)
> > > >
> > > > As suggested by Linus at [1]:
> > > > A dentry has a parent, but at the same time, a dentry really does
> > > > inherently have "one name" (and given just the dentry pointers, you
> > > > can't show mount-related parenthood, so in many ways the "show just
> > > > one name" makes sense for "%pd" in ways it doesn't necessarily for
> > > > "%pD"). But while a dentry arguably has that "one primary component",
> > > > a _file_ is certainly not exclusively about that last component.
> > > >
> > > > Hence "file_dentry_name()" simply shouldn't use "dentry_name()" at
> all.
> > > > Despite that shared code origin, and despite that similar letter
> > > > choice (lower-vs-upper case), a dentry and a file really are very
> > > > different from a name standpoint.
> > > >
> > > > Here stack space is preferred for file_d_path_name() because it is
> > > > much safer. The stack size 256 is a compromise between stack
> overflow
> > > > and too short full path.
> > >
> > > How is it "safer"?  You already have a buffer passed from the caller.
> > > Are you saying that d_path_fast() might overrun a really small buffer
> > > but won't overrun a 256 byte buffer?
> > No, it won't overrun a 256 byte buf. When the full path size is larger
> than 256, the p->len is < 0 in prepend_name, and this overrun will be
> > dectected in extract_string() with "-ENAMETOOLONG".
> >
> > Each printk contains 2 vsnprintf. vsnprintf() returns the required size
> after formatting the string.
> > 1. vprintk_store() will invoke 1st vsnprintf() will 8 bytes space to get
> the reserve_size. In this case, the _buf_ could be less than _end_ by
> design.
> > 2. Then it invokes 2nd printk_sprint()->vscnprintf()->vsnprintf() to
> really fill the space.
>
> I think you need to explain _that_ in the commit log, not make some
> nebulous claim of "safer".

Okay

>
> > If we choose the stack space, it can meet above 2 cases.
> >
> > If we pass the parameter like:
> > p = d_path_fast(path, buf, end - buf);
> > We need to handle the complicated logic in prepend_name()
> > I have tried this way in local test, the code logic is very complicated
> > and not so graceful.
> > e.g. I need to firstly go through the loop and get the full path size of
> > that file. And then return reserved_size for that 1st vsnprintf
>
> 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.

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).

This make things more complicated.

Hope I have described it clearly as above.

--
Cheers,
Justin (Jia He)


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