[Toybox] st_blocks

[enh]

On Mon, Sep 30, 2019 at 10:49 AM Rob Landley <rob at landley.net> wrote:
>
> On 9/28/19 11:46 AM, enh via Toybox wrote:
> > the only other idea i've had since: use perl or python to implement #4
> > inline. but there's no perl or python on Android devices, and there's
> > no way that i know of to SEEK_HOLE/SEEK_DATA in awk. so that seems
> > like a non-starter.
> >
> > On Thu, Sep 26, 2019 at 5:58 PM enh <enh at google.com> wrote:
> >>
> >> I shot myself in the foot with my tar test that checks both st_size and st_blocks (via stat(1)).
>
> I was thinking that might work in a mkroot qemu instance, but you have to
> control for filesystem.
>
> >> You may remember that I had to add a "don't run these tests on a system with
> >> SELinux xattrs" hack to the du tests. Basically, any test of st_blocks will be
> >> broken because of SELinux.
>
> Many things are. :)
>
> >> Unlike the du tests, I don't want to disable this tar sparse test unless I have
> >> to. But I don't have any non-terrible options.
> >>
> >> 1. Disable the test for SELinux.
> >>
> >> 2. Use shell arithmetic to allow some fudge.
> >>
> >> 3. Use setxattr to try to ensure that we're always using extra space, but then
> >> we need to work around failures on file systems without xattrs, and how much is enough, and...
>
> Run the test in a tmpfs mount with xattr support disabled? (I know you can do
> that at compile time, can you do it at runtime?)
>
> Or a loopback mounted ext2 that doesn't have xattr support, but again... I know
> how to remove it from the driver, I don't know how to prevent it at mount time.
> (I don't use selinux much.)
>
> Does vfat support xatrrs? If not, a vfat mount might also provide consistent
> results.
>
> But again, providing a mount point means having root to set stuff up. Hence
> mkroot...
>
> >> 4. Invent a new tool that actually dumps a map of where the holes are in a
> >> file. Unfortunately I'm not aware of any existing tool we could use, so
>
> We do have such a tool, it's called tar.
>
> >> then the test relies on toybox already being installed.
> >>
> >> 5. Use find -printf %S but because that's just using st_blocks behind the
> >> scenes, we'd need a fudge factor only now it would be a float instead of an
> >> int, which doesn't seem like an improvement.
> >>
> >> I haven't been able to come up with anything else, and I don't like any of
> >> those options, so ... anyone have a better idea?
>
> Create a reference tarball and see if extract-then-archive can recreate the
> reference tarball. (Or the first 2048 bytes of it, anyway.)

ah, yes. the same trick you've been using up and down that file :-)

attached.

> Rob
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