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Currently, TWRP backups don't include /data/media, and this doesn't appear to be on the developers' radar at all (the issue for it over at GitHub is just a bunch of +1s with no official response). Now that we have SELinux to contend with, simply copying the files back and forth is apparently no longer an option, and there are a number of different ways to access /data/media throughout the filesystem, each with different effective owners and permissions.

In excruciating detail, how should I approach backing up and restoring /data/media so everything is restored exactly as it was and my apps don't have trouble with the resulting permissions?

blackwind
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    Isn't /data/media just an alias for /storage/emulated/0? Permissions and ownership are always restored with Android .tar archives. – Aaron Gillion Mar 18 '16 at 07:25
  • It's not possible to dd /data/media since it's recognized as a directory. I've not tried, but I think that you can dd /dev/block/platform/msm_sdcc.1/by-name/userdata, instead. – Grimoire Mar 18 '16 at 14:19
  • @DeathMaskSalesman dd is surely a bad idea. It's even worse than TWRP & CWM's default backup method, tar. In case you're very wealthy and bought a 128GB phone and you only stored a few files in it, dd will still backup the whole partition, block by block, resulting in an image of 128GB in size. Also another serious problem is that you can't dd directly onto your computer, so you have to find another 128GB storage on your phone (SD card?) to hold the inage temporarily. – iBug Jan 22 '17 at 00:48
  • @iBug Of course it's a bad idea. If it were good, I would've posted an answer. By the way, you can dd directly to the computer by means of adb shell, so that the final command becomes adb shell "dd if=partition" > out.img. – Grimoire Jan 22 '17 at 01:19
  • @DeathMaskSalesman It still will fail. Root is required to dd from /dev/block. As often occurs, adbd cannot run as root in production builds. I just tested it out. However you can still do it in recovery but what's the point in doing so? – iBug Jan 22 '17 at 03:13
  • @iBug Well, one could mount the resulting image as a loop device, for "future" use. In my case, I did it to recover deleted files. – Grimoire Jan 22 '17 at 13:13
  • @DeathMaskSalesman Even in your case for further use, tar is still a better choice than dd. Also there hasn't been a solution to the storage problem discussed for dd's output image. I vote for tar. – iBug Jan 22 '17 at 15:58
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    @iBug So do I. Let me upvote your answer, by the way. – Grimoire Jan 22 '17 at 16:02
  • So, as far as I understand, that feature was excluded from TWRP just because toxic minimalists and confused inexperienced users are unable to use TWRP properly, making us power users suffer once again? – neverMind9 Apr 07 '19 at 12:40
  • According to the said GitHub comment by Dees-Troy, “[…] the average TWRP user can't even spell adb.” – Why do us power users have to suffer, just because some amateurs out there have no clue about how to use tools intended for power users? – neverMind9 Apr 07 '19 at 12:42
  • @neverMind9 "power user" will simply add bind mount /sdcard -> /storage in twrp.fstab (or adb pull) – alecxs Nov 11 '21 at 23:05

2 Answers2

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I recommend directly copying files from/to /sdcard.
The sdcard service will handle all the permissions and SELinux contexts for you.
The /storage/emulated/0, /mnt/shell/emulated/0 and /storage/emulated/legacy are all shadow reflexes of /data/media (typically /data/media/0). In the /init.rc file you can find out that the /system/bin/sdcard is a service that controls all of them, so you can just copy the files in /storage/emulated/0 with ease. All other directories like /sdcard and /storage/sdcard0 are symbolic links. They don't matter at all.

iBug
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I just adb pull /data/media/. media to copy the files to my pc adb directory. Then I do a TWRP backup everything to sdcard. Restore from sdcard backup, reboot recovery, then adb push media/. /data/media. Make certain to wipe sdcard from recovery, not formatted from pc. Works for me! Wrong format gives lots of error 255's.

keven
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