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

[Justin He]

Hi Petr

> -----Original Message-----
> From: Petr Mladek <pmladek@xxxxxxxx>
> Sent: Monday, May 10, 2021 9:05 PM
> To: Justin He <Justin.He@xxxxxxx>
> Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>; Sergey Senozhatsky
> <senozhatsky@xxxxxxxxxxxx>; Andy Shevchenko
> <andriy.shevchenko@xxxxxxxxxxxxxxx>; Rasmus Villemoes
> <linux@xxxxxxxxxxxxxxxxxx>; Jonathan Corbet <corbet@xxxxxxx>; Alexander
> Viro <viro@xxxxxxxxxxxxxxxxxx>; Linus Torvalds <torvalds@linux-
> foundation.org>; Al Viro <viro@xxxxxxxxxxxxxxxx>; Heiko Carstens
> <hca@xxxxxxxxxxxxx>; Vasily Gorbik <gor@xxxxxxxxxxxxx>; Christian
> Borntraeger <borntraeger@xxxxxxxxxx>; Eric W . Biederman
> <ebiederm@xxxxxxxxxxxx>; Darrick J. Wong <darrick.wong@xxxxxxxxxx>; Peter
> Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>; Ira Weiny <ira.weiny@xxxxxxxxx>;
> Eric Biggers <ebiggers@xxxxxxxxxx>; Ahmed S. Darwish
> <a.darwish@xxxxxxxxxxxxx>; linux-doc@xxxxxxxxxxxxxxx; linux-
> kernel@xxxxxxxxxxxxxxx; linux-s390@xxxxxxxxxxxxxxx; linux-
> fsdevel@xxxxxxxxxxxxxxx
> 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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