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    SubjectRE: [PATCH RFCv3 2/3] lib/vsprintf.c: make %pD print full path for file
    Date
    Hi Petr

    > -----Original Message-----
    > From: Petr Mladek <pmladek@suse.com>
    > Sent: Monday, June 14, 2021 11:41 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>; Peter Zijlstra (Intel) <peterz@infradead.org>; Eric
    > Biggers <ebiggers@google.com>; Ahmed S. Darwish <a.darwish@linutronix.de>;
    > linux-doc@vger.kernel.org; linux-kernel@vger.kernel.org; linux-
    > fsdevel@vger.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@mail.gmail.com/
    > >
    > > Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
    > > Signed-off-by: Jia He <justin.he@arm.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?
    >
    > > +
    > > + /* 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.
    >
    >
    > 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;

    As Rasmus confirmed that we needn't consider the spec.precision,
    the code can be more concise.
    I will send out v4 after testing together with one test_printf patch
    from Rasmus.


    --
    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

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