Hi Petr

-----Original Message-----
From: Petr Mladek <pmladek@suse.com>
Sent: Monday, June 14, 2021 11:41 PM
To: Justin He <redacted>
Cc: Steven Rostedt <rostedt@goodmis.org>; Sergey Senozhatsky
[off-list ref]; Andy Shevchenko
[off-list ref]; Rasmus Villemoes
[off-list ref]; Jonathan Corbet [off-list ref]; Alexander
Viro [off-list ref]; Linus Torvalds <torvalds@linux-
foundation.org>; Peter Zijlstra (Intel) [off-list ref]; Eric
Biggers [off-list ref]; Ahmed S. Darwish [off-list ref];
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:

quoted

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/

quoted

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Jia He <redacted>
---
 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

quoted

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

quoted

 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``

quoted

-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)

quoted

 }

 /* 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.

quoted

+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));

quoted

+
+		return widen_string(buf + full_len, full_len, end - lim,

spec);

quoted

+	}
+
+	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

quoted

 }

 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?

quoted

+
+	/* 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