RE: [PATCH RFCv4 2/4] lib/vsprintf.c: make '%pD' print full path for file

[Justin He]

Hi Petr

> -----Original Message-----
> From: Petr Mladek <pmladek@xxxxxxxx>
> Sent: Thursday, June 17, 2021 10:10 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>; Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>; Eric
> Biggers <ebiggers@xxxxxxxxxx>; Ahmed S. Darwish <a.darwish@xxxxxxxxxxxxx>;
> linux-doc@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; linux-
> fsdevel@xxxxxxxxxxxxxxx; Matthew Wilcox <willy@xxxxxxxxxxxxx>
> Subject: Re: [PATCH RFCv4 2/4] lib/vsprintf.c: make '%pD' print full path
> for file
>
> On Tue 2021-06-15 23:49:50, Jia He wrote:
> > Previously, the specifier '%pD' is for printing dentry name of struct
> > file. 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 change the behavior of '%pD' to print full path of that file.
> >
> > Precision is never going to be used with %p (or any of its kernel
> > extensions) if -Wformat is turned on.
> > .
> >
> > [1] https://lore.kernel.org/lkml/CAHk-=wimsMqGdzik187YWLb-
> ru+iktb4MYbMQG1rnZ81dXYFVg@xxxxxxxxxxxxxx/
> >
> > Suggested-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> > Signed-off-by: Jia He <justin.he@xxxxxxx>
>
> > --- a/lib/vsprintf.c
> > +++ b/lib/vsprintf.c
> > @@ -920,13 +921,41 @@ char *dentry_name(char *buf, char *end, const
> struct dentry *d, struct printf_sp
> >  }
> >
> >  static noinline_for_stack
> > -char *file_dentry_name(char *buf, char *end, const struct file *f,
> > +char *file_d_path_name(char *buf, char *end, const struct file *f,
> >                     struct printf_spec spec, const char *fmt)
> >  {
> > +   const struct path *path;
> > +   char *p;
> > +   int prepend_len, reserved_size, dpath_len;
> > +
> >     if (check_pointer(&buf, end, f, spec))
> >             return buf;
> >
> > -   return dentry_name(buf, end, f->f_path.dentry, spec, fmt);
> > +   path = &f->f_path;
> > +   if (check_pointer(&buf, end, path, spec))
> > +           return buf;
> > +
> > +   p = d_path_unsafe(path, buf, end - buf, &prepend_len);
> > +
> > +   /* Calculate the full d_path length, ignoring the tail '\0' */
> > +   dpath_len = end - buf - prepend_len - 1;
> > +
> > +   reserved_size = max_t(int, dpath_len, spec.field_width);
>
> "reserved_size" is kind of confusing. "dpath_widen_len" or just "widen_len"
> look much more obvious.

Okay

>
> The below comments are not bad. But they still made me thing about it
> more than I wanted ;-) I wonder if it following is better:
>
> > +   /* case 1: no space at all, forward the buf with reserved size */
> > +   if (buf >= end)
> > +           return buf + reserved_size;
>
>       /* Case 1: Already started past the buffer. Just forward @buf. */
>       if (buf >= end)
>               return buf + widen_len;
>
Okay
> > +
> > +   /*
> > +    * case 2: small scratch space for long d_path name. The space
> > +    * [buf,end] has been filled with truncated string. Hence use the
> > +    * full dpath_len for further string widening.
> > +    */
> > +   if (prepend_len < 0)
> > +           return widen_string(buf + dpath_len, dpath_len, end, spec);
>
>       /*
>        * Case 2: The entire remaining space of the buffer filled by
>        * the truncated path. Still need to get moved right when
>        * the filed width is greather than the full path length.
>        */
>       if (prepend_len < 0)
>               return widen_string(buf + dpath_len, dpath_len, end, spec);
>
Okay
> > +   /* case3: space is big enough */
> > +   return string_nocheck(buf, end, p, spec);
>
>       /*
>        * Case 3: The full path is printed at the end of the buffer.
>        * Print it at the right location in the same buffer.
>        */
>       return string_nocheck(buf, end, p, spec);
Okay
> >  }
> >  #ifdef CONFIG_BLOCK
> >  static noinline_for_stack
>
> In each case, I am happy that it was possible to simplify the logic.
> I got lost several times in the previous version.

Indeed, the cases can be much simpler if we don't consider spec.precision.
More than that, maybe we could remove the spec.precision consideration for
'%pd' or other pointer related specifiers also


--
Cheers,
Justin (Jia He)


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