support for Cygwin XATTRs in NTFS-3g?

[philcerf]

Hey.

Cygwin supports user.* XATTRs by encoding them somehow in Windows EAs.

Any chance to see support for reading these via ntfs-3g?

Maybe with default mount options, they could be mapped into some special namespace (like cygwin.*) and with some special mount option like xattr-profile=cygwin they could get mapped directly into user.* ?

I found only little information on how they do it.

There's a few words in https://www.cygwin.com/cygwin-ug-net/cygwin-ug-net.pdf on PDF page 4, chapter 1.6.3 File Access.

Other than that, one would probably have to resort to https://www.cygwin.com/git.html where it seems in

• winsup/cygwin/fhandler/disk_file.cc
• winsup/cygwin/include/attr/xattr.h
• winsup/cygwin/ntea.cc (which seems to contain the beef in read_ea())

ntfs-3g already has some limited support for getting the data:

$ getfattr -n system.ntfs_ea test --only-values | hd
00000000  90 00 00 00 00 04 80 00  46 4f 4f 4f 00 37 32 66  |........FOOO.72f|
00000010  32 35 64 39 30 65 66 34  63 66 65 63 64 61 38 66  |25d90ef4cfecda8f|
00000020  61 32 63 34 37 35 36 31  61 66 35 61 66 30 61 31  |a2c47561af5af0a1|
00000030  30 61 39 32 62 66 64 31  35 39 38 36 62 31 66 39  |0a92bfd15986b1f9|
00000040  31 36 33 35 38 62 66 36  61 63 38 61 33 37 38 35  |16358bf6ac8a3785|
00000050  38 61 31 34 64 32 37 33  32 39 35 30 36 61 33 37  |8a14d27329506a37|
00000060  36 36 62 61 64 30 66 33  34 64 32 65 30 34 63 61  |66bad0f34d2e04ca|
00000070  66 33 39 37 63 31 36 30  37 62 34 33 38 30 65 62  |f397c1607b4380eb|
00000080  33 33 63 39 37 64 33 37  64 66 63 33 37 00 00 00  |33c97d37dfc37...|
00000090

Here, on test the following XATTR was set from Cygwin:

$ setfattr -n user.fooo -v 72f25d90ef4cfecda8fa2c47561af5af0a10a92bfd15986b1f916358bf6ac8a37858a14d27329506a3766bad0f34d2e04caf397c1607b4380eb33c97d37dfc37 test

Not sure whether support for FAT and exFAT is needed, because it seems Cygwin itself cannot even set XATTRs on them.

Cheers,
Philippe