syscall: follow trusted sender ancestor links

[steadytao]

Fixes #1087

Summary

Explicit sender directory arguments now act as transfer roots and may pass through symlinks regardless of ownership. This fixes both absolute and relative source paths, including --relative while keeping recursive traversal confined beneath the selected source root.

Daemon paths, --files-from entries and file-leaf symlink handling are unchanged.

[steadytao]

@peabee Could you test this with your backup application using the original /mnt/home/My_Documents path? Please check that the backup completes and the expected files are copied.

[peabee]

Hi happy to test but unsure what "this" is or how to obtain it......
A binary would be ideal......
But if you want me to download source and compile I don't know how to download source with a pull request included - sorry!

[steadytao]

This is built against TrixiePup64, so it may not run on your Slackware-based Puppy - rsync.tar.gz

Make sure to keep your existing rsync; if this binary reports a library error, send me that error and your exact Puppy version/environment so I can provide a matching build.

[steadytao]

Ah and you can use git clone --single-branch --branch sender-ancestor-links https://github.com/RsyncProject/rsync.git to clone the PR if that is easier.

[seks99x]

Great job! there is still another case not covered. File transfers using the -R (--relative) flag are still broken across the board if a symlink is included in the path (full/relative paths). Directories or recursive transfers works well with full/relative paths.

┌──(root㉿192)-[~/rsync-latest-news]
└─# rsync/rsync -R rsync/lnk/test/a /home/omar/dest/ -> rel/full path case
rsync: [sender] send_files failed to open "/root/rsync-latest-news/rsync/lnk/test/a": Too many levels of symbolic links (40)
rsync error: some files/attrs were not transferred (see previous errors) (code 23) at main.c(1404) [sender=3.5.0-gd0a5ab7f]

Expected Behavior (rsync 3.4.1)
seks@omars-Mac-mini rsync-3.4.1 % ./rsync -R lnk/sub/a dest/ 
seks@omars-Mac-mini rsync-3.4.1 % ls dest/
lnk