Hi Rasmus > -----Original Message----- > From: Rasmus Villemoes <linux@xxxxxxxxxxxxxxxxxx> > Sent: Saturday, June 12, 2021 5:28 AM > To: Justin He <Justin.He@xxxxxxx>; Petr Mladek <pmladek@xxxxxxxx>; Steven > Rostedt <rostedt@xxxxxxxxxxx>; Sergey Senozhatsky > <senozhatsky@xxxxxxxxxxxx>; Andy Shevchenko > <andriy.shevchenko@xxxxxxxxxxxxxxx>; Jonathan Corbet <corbet@xxxxxxx>; > Alexander Viro <viro@xxxxxxxxxxxxxxxxxx>; Linus Torvalds <torvalds@linux- > foundation.org> > Cc: 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 11/06/2021 17.59, 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. */ > > +static char *string_truncate(char *buf, char *end, const char *s, > > + u32 full_len, struct printf_spec spec) > > +{ > > + int lim = 0; > > + > > + if (buf < end) { > > See below, I think the sole caller guarantees this, Ok, will remove this check statement > > > + 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; > > which is good because this would almost certainly be wrong (violating > the "always forward buf appropriately regardless of whether you wrote > something" rule). > > > +} > > 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); > > If I'm reading this right, you're using buf as scratch space to write > however much of the path fits. Then [*] > > > + /* 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; > > Why the !buf check? The only way we can have that is the snprintf(NULL, > 0, ...) case of asking how much space we'd need to malloc, right? In > which case end would be NULL+0 == NULL, so buf >= end automatically, > regardless of how much have been "printed" before %pD. My original purpose is to avoid any memory copy/move for kvasprintf-> vsnprintf(NULL, 0,...). But as you said, this can be folded into the case buf >= end. Do you think whether following case should be forbidden?: vsnprintf(NULL, 8,...). Sorry if it is too verbose. If above invoking is valid, !buf should still be checked. > > > + > > + /* small space for long name */ > > + if (buf < end && prepend_len < 0) > > So if we did an early return for buf >= end, we now know buf < end and > hence the first part here is redundant. > > Anyway, as for [*]: > > > + return string_truncate(buf, end, p, dpath_len, spec); > > + > > + /* space is enough */ > > + return string_nocheck(buf, end, p, spec); > > Now you're passing p to string_truncate or string_nocheck, while p > points somewhere into buf itself. I can't convince myself that would be > safe. At the very least, it deserves a couple of comments. When code goes here, the buffer space must be as follows: |.........|.........| buf p end So string_nocheck is safe because essential it would byte-to-byte copy p to buf. But I agree comments are needed here. -- Cheers, Justin (Jia He) IMPORTANT NOTICE: The contents of this email and any attachments are confidential and may also be privileged. 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