[PATCH v3 2/2] initramfs: introduce do_readxattrs()

[Roberto Sassu]

On 5/18/2019 12:17 AM, Arvind Sankar wrote:
> On Fri, May 17, 2019 at 02:47:31PM -0700, H. Peter Anvin wrote:
>> On 5/17/19 2:02 PM, Arvind Sankar wrote:
>>> On Fri, May 17, 2019 at 01:18:11PM -0700, hpa at zytor.com wrote:
>>>>
>>>> Ok... I just realized this does not work for a modular initramfs, composed at load time from multiple files, which is a very real problem. Should be easy enough to deal with: instead of one large file, use one companion file per source file, perhaps something like filename..xattrs (suggesting double dots to make it less likely to conflict with a "real" file.) No leading dot, as it makes it more likely that archivers will sort them before the file proper.
>>> This version of the patch was changed from the previous one exactly to deal with this case --
>>> it allows for the bootloader to load multiple initramfs archives, each
>>> with its own .xattr-list file, and to have that work properly.
>>> Could you elaborate on the issue that you see?
>>>
>>
>> Well, for one thing, how do you define "cpio archive", each with its own
>> .xattr-list file? Second, that would seem to depend on the ordering, no,
>> in which case you depend critically on .xattr-list file following the
>> files, which most archivers won't do.
>>
>> Either way it seems cleaner to have this per file; especially if/as it
>> can be done without actually mucking up the format.
>>
>> I need to run, but I'll post a more detailed explanation of what I did
>> in a little bit.
>>
>> 	-hpa
>>
> Not sure what you mean by how do I define it? Each cpio archive will
> contain its own .xattr-list file with signatures for the files within
> it, that was the idea.
> 
> You need to review the code more closely I think -- it does not depend
> on the .xattr-list file following the files to which it applies.
> 
> The code first extracts .xattr-list as though it was a regular file. If
> a later dupe shows up (presumably from a second archive, although the
> patch will actually allow a second one in the same archive), it will
> then process the existing .xattr-list file and apply the attributes
> listed within it. It then will proceed to read the second one and
> overwrite the first one with it (this is the normal behaviour in the
> kernel cpio parser). At the end once all the archives have been
> extracted, if there is an .xattr-list file in the rootfs it will be
> parsed (it would've been the last one encountered, which hasn't been
> parsed yet, just extracted).
> 
> Regarding the idea to use the high 16 bits of the mode field in
> the header that's another possibility. It would just require additional
> support in the program that actually creates the archive though, which
> the current patch doesn't.

Yes, for adding signatures for a subset of files, no changes to the ram
disk generator are necessary. Everything is done by a custom module. To
support a generic use case, it would be necessary to modify the
generator to execute getfattr and the awk script after files have been
placed in the temporary directory.

If I understood the new proposal correctly, it would be task for cpio to
read file metadata after the content and create a new record for each
file with mode 0x18000, type of metadata encoded in the file name and
metadata as file content. I don't know how easy it would be to modify
cpio. Probably the amount of changes would be reasonable.

The kernel will behave in a similar way. It will call do_readxattrs() in
do_copy() for each file. Since the only difference between the current
and the new proposal would be two additional calls to do_readxattrs() in
do_name() and unpack_to_rootfs(), maybe we could support both.

Roberto

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