Message-ID: <AM6PR08MB4376D26EFD5886B7CF21EFCFF7309@AM6PR08MB4376.eurprd08.prod.outlook.com>
Date:   Tue, 15 Jun 2021 06:48:24 +0000
From:   Justin He <Justin.He@....com>
To:     Petr Mladek <pmladek@...e.com>
CC:     Steven Rostedt <rostedt@...dmis.org>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Jonathan Corbet <corbet@....net>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Eric Biggers <ebiggers@...gle.com>,
        "Ahmed S. Darwish" <a.darwish@...utronix.de>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>
Subject: RE: [PATCH RFCv3 2/3] lib/vsprintf.c: make %pD print full path for
 file

Hi Petr

> -----Original Message-----
> From: Petr Mladek <pmladek@...e.com>
> Sent: Monday, June 14, 2021 11:41 PM
> To: Justin He <Justin.He@....com>
> Cc: Steven Rostedt <rostedt@...dmis.org>; Sergey Senozhatsky
> <senozhatsky@...omium.org>; Andy Shevchenko
> <andriy.shevchenko@...ux.intel.com>; Rasmus Villemoes
> <linux@...musvillemoes.dk>; Jonathan Corbet <corbet@....net>; Alexander
> Viro <viro@...iv.linux.org.uk>; Linus Torvalds <torvalds@...ux-
> foundation.org>; Peter Zijlstra (Intel) <peterz@...radead.org>; Eric
> Biggers <ebiggers@...gle.com>; Ahmed S. Darwish <a.darwish@...utronix.de>;
> linux-doc@...r.kernel.org; linux-kernel@...r.kernel.org; linux-
> fsdevel@...r.kernel.org
> Subject: Re: [PATCH RFCv3 2/3] lib/vsprintf.c: make %pD print full path for
> file
>
> On Fri 2021-06-11 23:59:52, 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 change the behavior of '%pD' to print full path of that file.
> >
> > Things become more complicated when spec.precision and spec.field_width
> > is added in. string_truncate() is to handle the small space case for
> > '%pD' precision and field_width.
> >
> > [1] https://lore.kernel.org/lkml/CAHk-=wimsMqGdzik187YWLb-
> ru+iktb4MYbMQG1rnZ81dXYFVg@...l.gmail.com/
> >
> > Suggested-by: Linus Torvalds <torvalds@...ux-foundation.org>
> > Signed-off-by: Jia He <justin.he@....com>
> > ---
> >  Documentation/core-api/printk-formats.rst |  5 ++-
> >  lib/vsprintf.c                            | 47 +++++++++++++++++++++--
> >  2 files changed, 46 insertions(+), 6 deletions(-)
> >
> > diff --git a/Documentation/core-api/printk-formats.rst
> b/Documentation/core-api/printk-formats.rst
> > index f063a384c7c8..95ba14dc529b 100644
> > --- a/Documentation/core-api/printk-formats.rst
> > +++ b/Documentation/core-api/printk-formats.rst
> > @@ -408,12 +408,13 @@ dentry names
> >  ::
> >
> >     %pd{,2,3,4}
> > -   %pD{,2,3,4}
> > +   %pD
> >
> >  For printing dentry name; if we race with :c:func:`d_move`, the name
> might
> >  be a mix of old and new ones, but it won't oops.  %pd dentry is a safer
> >  equivalent of %s dentry->d_name.name we used to use, %pd<n> prints ``n``
> > -last components.  %pD does the same thing for struct file.
> > +last components.  %pD prints full file path together with mount-related
> > +parenthood.
> >
> >  Passed by reference.
> >
> > diff --git a/lib/vsprintf.c b/lib/vsprintf.c
> > index f0c35d9b65bf..317b65280252 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>
> > @@ -601,6 +602,20 @@ char *widen_string(char *buf, int n, char *end,
> struct printf_spec spec)
> >  }
> >
> >  /* Handle string from a well known address. */
>
> This comment is for widen_string().
>
> string_truncate() functionality is far from obvious. It would deserve
> it's own description, including description of each parammeter.
>
> Well, do we really need it? See below.
>
> > +static char *string_truncate(char *buf, char *end, const char *s,
> > +                        u32 full_len, struct printf_spec spec)
> > +{
> > +   int lim = 0;
> > +
> > +   if (buf < end) {
> > +           if (spec.precision >= 0)
> > +                   lim = strlen(s) - min_t(int, spec.precision, strlen(s));
> > +
> > +           return widen_string(buf + full_len, full_len, end - lim, spec);
> > +   }
> > +
> > +   return buf;
> > +}
> >  static char *string_nocheck(char *buf, char *end, const char *s,
> >                         struct printf_spec spec)
> >  {
> > @@ -920,13 +935,37 @@ 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);
> > +
> > +   /* Minus 1 byte for '\0' */
> > +   dpath_len = end - buf - prepend_len - 1;
> > +
> > +   reserved_size = max_t(int, dpath_len, spec.field_width);
> > +
> > +   /* no filling space at all */
> > +   if (buf >= end || !buf)
> > +           return buf + reserved_size;
> > +
> > +   /* small space for long name */
> > +   if (buf < end && prepend_len < 0)
> > +           return string_truncate(buf, end, p, dpath_len, spec);
>
> We need this only because we allowed to write the path behind
> spec.field_width. Do I get it right?

Both of field_width and precision:
"%.14pD" or "%8.14pD"

>
> > +
> > +   /* space is enough */
> > +   return string_nocheck(buf, end, p, spec);
> >  }
>
> It easy to get lost in all the computations, including the one
> in string_truncate():
>
>       dpath_len = end - buf - prepend_len - 1;
>       reserved_size = max_t(int, dpath_len, spec.field_width);
> and
>       lim = strlen(s) - min_t(int, spec.precision, strlen(s));
>       return widen_string(buf + full_len, full_len, end - lim, spec);
>
> Please, add comments explaining the meaning of the variables a bit.
> They should help to understand why it is done this way.
>
Sure, sorry about that
>
> I tried another approach below. The main trick is that
> max_len is limited by spec.field_width and spec.precision before calling
> d_path_unsave():
>
>
>       if (check_pointer(&buf, end, f, spec))
>               return buf;
>
>       path = &f->f_path;
>       if (check_pointer(&buf, end, path, spec))
>               return buf;
>
>       max_len = end - buf;
>       if (spec.field_width >= 0 && spec.field_width < max_len)
>               max_len = spec.filed_width;
>       if (spec.precision >= 0 && spec.precision < max_len)
>               max_len = spec.precision;
>
>       p = d_path_unsafe(path, buf, max_len, &prepend_len);
>
>       /*
>        * The path has been printed from the end of the buffer.
>        * Process it like a normal string to handle "precission"
>        * and "width" effects. In the "worst" case, the string
>        * will stay as is.
>        */
>       if (buf < end) {
>               buf = string_nocheck(buf, end, p, spec);
>               /* Return buf when output was limited or did fit in. */
>               if (spec.field_width >= 0 || spec.precision >= 0 ||
>                   prepend_len >= 0) {
>                       return buf;
>               }
>               /* Otherwise, add what was missing. Ignore tail '\0' */
>               return buf - prepend_len - 1;
>       }
>
>       /*
>        * Nothing has been written to the buffer. Just count the length.
>        * I is fixed when field_with is defined. */
>       if (spec.field_width >= 0)
>               return buf + spec.field_width;
>
>       /* Otherwise, use the length of the path. */
>       dpath_len = max_len - prepend_len - 1;
>
>       /* The path might still get limited by precision number. */
>       if (spec.precision >= 0 && spec.precision < dpath_len)
>               return buf + spec.precision;
>
>       return buf + dpath_len;
>

Let me check it carefully, thanks for your suggestion.


--
Cheers,
Justin (Jia He)


>
> Note that the above code is not even compile tested. There might be
> off by one mistakes. Also, it is possible that I missed something.
>
> Best Regards,
> Petr
IMPORTANT NOTICE: The contents of this email and any attachments are confidential and may also be privileged. If you are not the intended recipient, please notify the sender immediately and do not disclose the contents to any other person, use it for any purpose, or store or copy the information in any medium. Thank you.

Powered by blists - more mailing lists