Re: [PATCH] generic/453: Exclude filenames that are not supported by exfat

[Shreeya Patel <shreeya.patel@collabora.com>]

On 26/04/21 6:07 pm, Matthew Wilcox wrote:
> On Mon, Apr 26, 2021 at 05:27:51PM +0530, Shreeya Patel wrote:
>> On 26/04/21 6:04 am, Matthew Wilcox wrote:
>>> On Mon, Apr 26, 2021 at 04:01:05AM +0530, Shreeya Patel wrote:
>>>> exFAT filesystem does not support the following character codes
>>>> 0x0000 - 0x001F ( Control Codes ), /, ?, :, ", \, *, <, |, >
>>> ummm ...
>>>
>>>> -# Fake slash?
>>>> -setf "urk\xc0\xafmoo" "FAKESLASH"
>>> That doesn't use any of the explained banned characters.  It uses 0xc0,
>>> 0xaf.
>>>
>>> Now, in utf-8, that's an nonconforming sequence.  "The Unicode and UCS
>>> standards require that producers of UTF-8 shall use the shortest form
>>> possible, for example, producing a two-byte sequence with first byte 0xc0
>>> is nonconforming.  Unicode 3.1 has added the requirement that conforming
>>> programs must not accept non-shortest forms in their input."
>>>
>>> So is it that exfat is rejecting nonconforming sequences?  Or is it
>>> converting the nonconforming sequence from 0xc0 0xaf to the conforming
>>> sequence 0x2f, and then rejecting it (because it's '/')?
>>>
>> No, I don't think exfat is not converting nonconforming sequence from 0xc0
>> 0xaf
>> to the conforming sequence 0x2f.
>> Because I get different outputs when tried with both ways.
>> When I create a file with "urk\xc0\xafmoo", I get output as "Operation not
>> permitted"
>> and when I create it as "urk\x2fmoo", it gives "No such file or directory
>> error" or
>> you can consider this error as "Invalid argument"
>> ( because that's what I get when I try for other characters like |, :, ?,
>> etc )
> I think we need to understand this before skipping the test.  Does it
> also fail, eg, on cifs, vfat, jfs or udf?


I tested it for VFAT, UDF and JFS and following are the results.


1. VFAT ( as per wikipedia 0x00-0x1F 0x7F " * / : < > ? \ | are reserved 
characters)

For \x2f - /var/mnt/scratch/test-453/urk/moo.txt: No such file or directory

For \xc0\xaf) - /var/mnt/scratch/test-453/urk��moo.txt: Invalid argument

Also gives error for Box filename

( this is very much similar to exfat, the only difference is that I do 
not get Operation not permitted when
using \xc0\xaf, instead it gives invalid argument.)


2. UDF ( as per wikipedia - only NULL cannot be used )

For \x2f - /var/mnt/scratch/test-453/urk/moo.txt: No such file or directory

For \xc0\xaf - creates filename something like this 'urk??moo.txt' and 
does not throw any error.
( But this seems to be invalid and should have thrown some error)

Also gives error for dotdot entry.

I am not sure why UDF was giving error for / and dot dot entry but then
I read the following for UDF in one of the man pages which justifies the 
above errors I think

"Invalid characters such as "NULL" and "/" and  invalid  file
names  such  as "." and ".." will be translated according to
the following rule:

Replace the invalid character with an "_," then  append  the
file name with # followed by a 4 digit hex representation of
the 16-bit CRC of the original FileIdentifier. For  example,
the file name ".." will become "__#4C05" "

Source - http://www-it.desy.de/cgi-bin/man-cgi?udfs+7


3. JFS ( as per Wikipedia NULL cannot be used )

For \x2f - /var/mnt/scratch/test-453/urk/moo.txt: No such file or directory

For \xc0\xaf - Works fine

Again not sure why / is failing here. Did not find much resource about 
the restricted filenames for JFS.


So as per above all the results, it seems like using \x2f fails for all 
but \xc0\xaf does work for JFS.


>
>> Box filename also fails with "Invalid argument" error.
>>
>>