Hi Petr > -----Original Message----- > From: Petr Mladek <pmladek@xxxxxxxx> > Sent: Monday, June 14, 2021 11:41 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 > 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@xxxxxxxxxxxxxx/ > > > > Suggested-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> > > Signed-off-by: Jia He <justin.he@xxxxxxx> > > --- > > 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
