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

Justin He
2021-05-10T14:28:50Z

Hi Petr

> -----Original Message-----
> From: Petr Mladek <pmladek@suse.com>
> Sent: Monday, May 10, 2021 9:05 PM
> To: Justin He <Justin.He@arm.com>
> Cc: 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>; Al Viro <viro@ftp.linux.org.uk>; Heiko Carstens
> <hca@linux.ibm.com>; Vasily Gorbik <gor@linux.ibm.com>; Christian
> Borntraeger <borntraeger@de.ibm.com>; Eric W . Biederman
> <ebiederm@xmission.com>; Darrick J. Wong <darrick.wong@oracle.com>; Peter
> Zijlstra (Intel) <peterz@infradead.org>; Ira Weiny <ira.weiny@intel.com>;
> Eric Biggers <ebiggers@google.com>; Ahmed S. Darwish
> <a.darwish@linutronix.de>; linux-doc@vger.kernel.org; linux-
> kernel@vger.kernel.org; linux-s390@vger.kernel.org; linux-
> fsdevel@vger.kernel.org
> Subject: Re: [PATCH RFC 2/3] lib/vsprintf.c: make %pD print full path for
> file
>
> On Sat 2021-05-08 20:25:29, 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.
> >
> > diff --git a/lib/vsprintf.c b/lib/vsprintf.c
> > index f0c35d9b65bf..8220ab1411c5 100644
> > --- a/lib/vsprintf.c
> > +++ b/lib/vsprintf.c
> > @@ -27,6 +27,7 @@
> > #include <linux/string.h>
> > #include <linux/ctype.h>
> > #include <linux/kernel.h>
> > +#include <linux/dcache.h>
> > #include <linux/kallsyms.h>
> > #include <linux/math64.h>
> > #include <linux/uaccess.h>
> > @@ -923,10 +924,17 @@ static noinline_for_stack
> > char *file_dentry_name(char *buf, char *end, const struct file *f,
> > struct printf_spec spec, const char *fmt)
> > {
> > + const struct path *path = &f->f_path;
>
> This dereferences @f before it is checked by check_pointer().

Okay

>
> > + char *p;
> > + char tmp[128];
> > +
> > if (check_pointer(&buf, end, f, spec))
> > return buf;
> >
> > - return dentry_name(buf, end, f->f_path.dentry, spec, fmt);
> > + p = d_path_fast(path, (char *)tmp, 128);
> > + buf = string(buf, end, p, spec);
>
> Is 128 a limit of the path or just a compromise, please?

It is just a compromise.

>
> d_path_fast() limits the size of the buffer so we could use @buf
> directly. We basically need to imitate what string_nocheck() does:
>
> + the length is limited by min(spec.precision, end-buf);
> + the string need to get shifted by widen_string()
>
> We already do similar thing in dentry_name(). It might look like:
>
> char *file_dentry_name(char *buf, char *end, const struct file *f,
> struct printf_spec spec, const char *fmt)
> {
> const struct path *path;
> int lim, len;
> char *p;
>
> if (check_pointer(&buf, end, f, spec))
> return buf;
>
> path = &f->f_path;
> if (check_pointer(&buf, end, path, spec))
> return buf;
>
> lim = min(spec.precision, end - buf);
> p = d_path_fast(path, buf, lim);
> if (IS_ERR(p))
> return err_ptr(buf, end, p, spec);
>
> len = strlen(buf);
> return widen_string(buf + len, len, end, spec);
> }
>
> Note that the code is _not_ even compile tested. It might include
> some ugly mistake.

Okay, let me try it together with Linus's prepend_entries().
Thanks for the suggestion.
>
> > +
> > + return buf;
> > }
> > #ifdef CONFIG_BLOCK
> > static noinline_for_stack
> > @@ -2296,7 +2304,7 @@ early_param("no_hash_pointers",
> no_hash_pointers_enable);
> > * - 'a[pd]' For address types [p] phys_addr_t, [d] dma_addr_t and
> derivatives
> > * (default assumed to be phys_addr_t, passed by reference)
> > * - 'd[234]' For a dentry name (optionally 2-4 last components)
> > - * - 'D[234]' Same as 'd' but for a struct file
> > + * - 'D' Same as 'd' but for a struct file
>
> It is not really the same. We should make it clear that it prints
> the full path:

Okay

--
Cheers,
Justin (Jia He)

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