Re: adding linux syscall fallocate

[r0ller <r0ller%freemail.hu@localhost>]

Hi Maciej,

Thanks for the detailed answer! Unfortunately, I don't think that I could accomplish this task in my spare time:(

Please, don't take this as an offence but as a fix for my case I thought of an utter hack:

I do know that writing a file by calling fallocate can be tricked and redirected into a named pipe of which the data can simply be pulled into a normal file. This is what I'm already doing in my project as a workaround when building it as 32bit arm lib:

mkfifo mypipe
cat mypipe > myfile
<execute 32bit crosscompile via ndk>

The problem with this is that it cannot be used when crosscompiling autoconf projects where a configure script starts creating many files as I'd need to edit the script at too many places to implement this trick.

However, if I could carry out this trick with the pipe when intercepting the linux fallocate call, it could work. Do you think it feasible?

Best regards,
r0ller

-------- Eredeti levél --------
Feladó: Maciej < maciej.grochowski%protonmail.com@localhost (Link -> mailto:maciej.grochowski%protonmail.com@localhost) >
Dátum: 2019 november 4 23:32:56
Tárgy: Re: adding linux syscall fallocate
Címzett: r0ller < r0ller%freemail.hu@localhost (Link -> mailto:r0ller%freemail.hu@localhost) >
 
Hi r0ller,
 
A couple of weeks ago I also run to the issue when I found lack of fallocate or POSIX_FALLOCATE(2) (to be precise) a little bit sad.
>From the very first view typical usage of POSIX_FALLOCATE(2) seems straight forward, comparing to the Linux fallocate(2) where different modes have to be supported. However, things can go a little bit more complicated if you are dealing with an existing file with a more complex structure.
Because of that, I found difficult to provide good quality implementation without a proper way to test it.
Before EuroBSD 2019 as a part of work about fuzzing the FFS, I decided to bring some well known FS test (namely speaking "XFS tests') suit to make sure that bugs that we fix did not introduce a regression.
The same thing applies to the new features of FS, is relatively easy to port implementation from other OS/FS, but without a proper way to test them, I would be very careful to introduce such things too quickly to the end-users.
 
One thing that I was missing for XFS tests, and going to publish part of it soon, is a way to view the internal structure of inodes and other metadata of Filesystem. My primary use case was to debug mount issues, in the example the issue that I showed during my presentation about the fuzzing. But also same apply to the code that manipulates inode blocks.
 
Hopefully, we can elaborate on that, and as I pointed earlier I would be careful with merging new FS features especially such wide used as POSIX_FALLOCATE(2) without a proper FS test suit or extensive enough testing that would require proper too like i.e. FSDB.
 
Thanks
Maciej
 
‐‐‐‐‐‐‐ Original Message ‐‐‐‐‐‐‐
On Sunday, November 3, 2019 6:06 PM, r0ller <r0ller%freemail.hu@localhost> wrote:
 
Hi Jaromir,
 
Indeed. That's bad news but thanks for your answer! I've even found this: https://wiki.netbsd.org/projects/project/ffs-fallocate/
Are there any details for this project besides that page? I don't know anything about NetBSD internals though if it's not meant for gurus, I'd have a look at it and give it a try.
 
Best regards,
r0ller
 
-------- Eredeti levél --------
Feladó: Jaromír Doleček < jaromir.dolecek%gmail.com@localhost (Link -> mailto:jaromir.dolecek%gmail.com@localhost) >
Dátum: 2019 november 3 15:16:34
Tárgy: Re: adding linux syscall fallocate
Címzett: r0ller < r0ller%freemail.hu@localhost (Link -> mailto:r0ller%freemail.hu@localhost) >
Le dim. 3 nov. 2019 à 08:57, r0ller <r0ller%freemail.hu@localhost> a écrit :
 
> As you can see on the attached screenshot, "line 4741" gets printed out. So I went on to check what happens in VOP_FALLOCATE but it gets really internal there.
>
> Does anyone have any hint?
 
fallocate VOP is not implemented for FFS:
 
> grep fallocate *
ffs_vnops.c: { &vop_fallocate_desc, genfs_eopnotsupp }, /* fallocate */
ffs_vnops.c: { &vop_fallocate_desc, spec_fallocate }, /* fallocate */
ffs_vnops.c: { &vop_fallocate_desc, vn_fifo_bypass }, /* fallocate */
 
Jaromir