On Thursday 27 March 2025 21:57:34 Amir Goldstein wrote: > On Thu, Mar 27, 2025 at 8:26 PM Pali Rohár <p...@kernel.org> wrote: > > > > On Thursday 27 March 2025 12:47:02 Amir Goldstein wrote: > > > On Sun, Mar 23, 2025 at 11:32 AM Pali Rohár <p...@kernel.org> wrote: > > > > > > > > On Sunday 23 March 2025 09:45:06 Amir Goldstein wrote: > > > > > On Fri, Mar 21, 2025 at 8:50 PM Andrey Albershteyn > > > > > <aalbe...@redhat.com> wrote: > > > > > > > > > > > > This patchset introduced two new syscalls getfsxattrat() and > > > > > > setfsxattrat(). These syscalls are similar to FS_IOC_FSSETXATTR > > > > > > ioctl() > > > > > > except they use *at() semantics. Therefore, there's no need to open > > > > > > the > > > > > > file to get an fd. > > > > > > > > > > > > These syscalls allow userspace to set filesystem inode attributes on > > > > > > special files. One of the usage examples is XFS quota projects. > > > > > > > > > > > > XFS has project quotas which could be attached to a directory. All > > > > > > new inodes in these directories inherit project ID set on parent > > > > > > directory. > > > > > > > > > > > > The project is created from userspace by opening and calling > > > > > > FS_IOC_FSSETXATTR on each inode. This is not possible for special > > > > > > files such as FIFO, SOCK, BLK etc. Therefore, some inodes are left > > > > > > with empty project ID. Those inodes then are not shown in the quota > > > > > > accounting but still exist in the directory. This is not critical > > > > > > but in > > > > > > the case when special files are created in the directory with > > > > > > already > > > > > > existing project quota, these new inodes inherit extended > > > > > > attributes. > > > > > > This creates a mix of special files with and without attributes. > > > > > > Moreover, special files with attributes don't have a possibility to > > > > > > become clear or change the attributes. This, in turn, prevents > > > > > > userspace > > > > > > from re-creating quota project on these existing files. > > > > > > > > > > > > Christian, if this get in some mergeable state, please don't merge > > > > > > it > > > > > > yet. Amir suggested these syscalls better to use updated struct > > > > > > fsxattr > > > > > > with masking from Pali Rohár patchset, so, let's see how it goes. > > > > > > > > > > Andrey, > > > > > > > > > > To be honest I don't think it would be fair to delay your syscalls > > > > > more > > > > > than needed. > > > > > > > > I agree. > > > > > > > > > If Pali can follow through and post patches on top of your syscalls > > > > > for > > > > > next merge window that would be great, but otherwise, I think the > > > > > minimum requirement is that the syscalls return EINVAL if fsx_pad > > > > > is not zero. we can take it from there later. > > > > > > > > IMHO SYS_getfsxattrat is fine in this form. > > > > > > > > For SYS_setfsxattrat I think there are needed some modifications > > > > otherwise we would have problem again with backward compatibility as > > > > is with ioctl if the syscall wants to be extended in future. > > > > > > > > I would suggest for following modifications for SYS_setfsxattrat: > > > > > > > > - return EINVAL if fsx_xflags contains some reserved or unsupported flag > > > > > > > > - add some flag to completely ignore fsx_extsize, fsx_projid, and > > > > fsx_cowextsize fields, so SYS_setfsxattrat could be used just to > > > > change fsx_xflags, and so could be used without the preceding > > > > SYS_getfsxattrat call. > > > > > > > > What do you think about it? > > > > > > I think all Andrey needs to do now is return -EINVAL if fsx_pad is not > > > zero. > > > > > > You can use this later to extend for the semantics of flags/fields mask > > > and we can have a long discussion later on what this semantics should be. > > > > > > Right? > > > > > > Amir. > > > > It is really enough? > > I don't know. Let's see... > > > All new extensions later would have to be added > > into fsx_pad fields, and currently unused bits in fsx_xflags would be > > unusable for extensions. > > I am working under the assumption that the first extension would be > to support fsx_xflags_mask and from there, you could add filesystem > flags support checks and then new flags. Am I wrong? > > Obviously, fsx_xflags_mask would be taken from fsx_pad space. > After that extension is implemented, calling SYS_setfsxattrat() with > a zero fsx_xflags_mask would be silly for programs that do not do > the legacy get+set. > > So when we introduce fsx_xflags_mask, we could say that a value > of zero means that the mask is not being checked at all and unknown > flags in set syscall are ignored (a.k.a legacy ioctl behavior). > > Programs that actually want to try and set without get will have to set > a non zero fsx_xflags_mask to do something useful.
Here we need to also solve the problem that without GET call we do not have valid values for fsx_extsize, fsx_projid, and fsx_cowextsize. So maybe we would need some flag in fsx_pad that fsx_extsize, fsx_projid, or fsx_cowextsize are ignored/masked. > I don't think this is great. > I would rather that the first version of syscalls will require the mask > and will always enforce filesystems supported flags. It is not great... But what about this? In a first step (part of this syscall patch series) would be just a check that fsx_pad is zero. Non-zero will return -EINVAL. In next changes would added fsx_filter bit field, which for each fsx_xflags and also for fsx_extsize, fsx_projid, and fsx_cowextsize fields would add a new bit flag which would say (when SET) that the particular thing has to be ignored. So when fsx_pad is all-zeros then fsx_filter (first field in fsx_pad) would say that nothing in fsx_xflags, fsx_extsize, fsx_projid, and fsx_cowextsize is ignored, and hence behave like before. And when something in fsx_pad/fsx_filter is set then it says which fields are ignored/filtered-out. > If you can get those patches (on top of current series) posted and > reviewed in time for the next merge window, including consensus > on the actual semantics, that would be the best IMO. I think that this starting to be more complicated to rebase my patches in a way that they do not affect IOCTL path but implement it properly for new syscall path. It does not sounds like a trivial thing which I would finish in merge window time and having proper review and consensus on this. > But I am just preparing a plan B in case you do not have time to > work on the patches or if consensus on the API extensions is not > reached on time. > > I think that for plan B, the minimum is to verify zero pad field and > that is something that this syscall has to do anyway, because this > is the way that backward compact APIs work. > > If you want the syscall to always return -EINVAL for setting xflags > that are currently undefined I agree that would be nice as well. > > Thanks, > Amir.
