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 9:00 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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> 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 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.

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

Thanks for any suggestion

--
Cheers,
Justin (Jia He)

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