On Tue, Jun 01, 2021 at 07:10:41PM +0300, Andy Shevchenko wrote:

On Tue, Jun 01, 2021 at 06:53:26PM +0300, Andy Shevchenko wrote:

quoted

On Tue, Jun 01, 2021 at 04:44:00PM +0100, Matthew Wilcox wrote:

quoted

On Tue, Jun 01, 2021 at 06:36:41PM +0300, Andy Shevchenko wrote:

quoted

On Tue, Jun 1, 2021 at 6:32 PM Matthew Wilcox [off-list ref] wrote:

quoted

On Tue, Jun 01, 2021 at 02:42:15PM +0000, Justin He wrote:

...

quoted

Just don't put anything
in the buffer if the user didn't supply enough space.  As long as you
get the return value right, they know the string is bad (or they don't
care if the string is bad)

It might be that I'm out of context here, but printf() functionality
in the kernel (vsprintf() if being precise)  and its users consider
that it should fill buffer up to the end of whatever space is
available.

Do they though?  What use is it to specify a small buffer, print a
large filename into it and then use that buffer, knowing that it wasn't
big enough?  That would help decide whether we should print the
start or the end of the filename.

Remember, we're going for usefulness here, not abiding by the letter of
the standard under all circumstances, no matter the cost.  At least
partially because we're far outside the standard here; POSIX does
not specify what %pD does.

"The argument shall be a pointer to void. The value of the
pointer is converted to a sequence of printable characters, in an
implementation-defined manner."

All nice words, but don't forget kasprintf() or other usages like this.
For the same input we have to have the same result independently on the room in
the buffer.

So, if I print "Hello, World" I should always get it, not "Monkey's Paw".
I.o.w.

 snprintf(10) ==> "Hello, Wor"
 snprintf(5)  ==> "Hello"
 snprintf(2)  !=> "Mo"
 snprintf(1)  !=> "M"
 snprintf(1)  ==> "H"

Inconsistency here is really not what we want.

I have to add that in light of the topic those characters should be counted
from the end of the filename. So, we will give user as much as possible of useful
information. I.o.w. always print the last part of filename up to the buffer
size or if the filename is shorter than buffer we will have it in full.

Ah, not monkey's paw, but donkey hoof then ...

Here's some examples, what do you think makes sense?

snprintf(buf, 16, "bad file '%pD'\n", q);

what content do you want buf to have when q is variously:

1. /abcd/efgh
2. /a/bcdefgh.iso
3. /abcdef/gh

I would argue that
"bad file ''\n"
is actually a better string to have than any of (case 2)
"bad file '/a/bc"
"bad file 'bcdef"
"bad file 'h.iso"