Re: [PATCH RFC 2/3] lib/vsprintf.c: make %pD print full path for file

Petr Mladek
2021-05-10T13:40:50Z

On Sat 2021-05-08 20:25:29, 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 "file_dentry_name()" simply shouldn't use "dentry_name()" at all.
> Despite that shared code origin, and despite that similar letter
> choice (lower-vs-upper case), a dentry and a file really are very
> different from a name standpoint.
>
> diff --git a/lib/vsprintf.c b/lib/vsprintf.c
> index f0c35d9b65bf..8220ab1411c5 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>
> @@ -923,10 +924,17 @@ static noinline_for_stack
> char *file_dentry_name(char *buf, char *end, const struct file *f,
> struct printf_spec spec, const char *fmt)
> {
> + const struct path *path = &f->f_path;

This dereferences @f before it is checked by check_pointer().

> + char *p;
> + char tmp[128];
> +
> if (check_pointer(&buf, end, f, spec))
> return buf;
>
> - return dentry_name(buf, end, f->f_path.dentry, spec, fmt);
> + p = d_path_fast(path, (char *)tmp, 128);
> + buf = string(buf, end, p, spec);

Is 128 a limit of the path or just a compromise, please?

d_path_fast() limits the size of the buffer so we could use @buf
directly. We basically need to imitate what string_nocheck() does:

+ the length is limited by min(spec.precision, end-buf);
+ the string need to get shifted by widen_string()

We already do similar thing in dentry_name(). It might look like:

char *file_dentry_name(char *buf, char *end, const struct file *f,
struct printf_spec spec, const char *fmt)
{
const struct path *path;
int lim, len;
char *p;

if (check_pointer(&buf, end, f, spec))
return buf;

path = &f->f_path;
if (check_pointer(&buf, end, path, spec))
return buf;

lim = min(spec.precision, end - buf);
p = d_path_fast(path, buf, lim);
if (IS_ERR(p))
return err_ptr(buf, end, p, spec);

len = strlen(buf);
return widen_string(buf + len, len, end, spec);
}

Note that the code is _not_ even compile tested. It might include
some ugly mistake.

> +
> + return buf;
> }
> #ifdef CONFIG_BLOCK
> static noinline_for_stack
> @@ -2296,7 +2304,7 @@ early_param("no_hash_pointers", no_hash_pointers_enable);
> * - 'a[pd]' For address types [p] phys_addr_t, [d] dma_addr_t and derivatives
> * (default assumed to be phys_addr_t, passed by reference)
> * - 'd[234]' For a dentry name (optionally 2-4 last components)
> - * - 'D[234]' Same as 'd' but for a struct file
> + * - 'D' Same as 'd' but for a struct file

It is not really the same. We should make it clear that it prints
the full path:

+ * - 'D' Same as 'd' but for a struct file; prints full path with
+ the mount-related parenthood

> * - 'g' For block_device name (gendisk + partition number)
> * - 't[RT][dt][r]' For time and date as represented by:
> * R struct rtc_time
> --
> 2.17.1

Best Regards,
Petr