APFS synthetic symbolic link traversing broken

Originator:gjasny
Number:rdar://FB9341328 Date Originated:16.06.2021
Status:Open Resolved:
Product:macOS Product Version:11.4 (build 20F71)
Classification: Reproducible:yes
 
Hello,

This is a bug report about synthetic symbolic links in APFS.

I investigated the following CMake issue where I noticed that something is broken at the macOS kernel level:
https://gitlab.kitware.com/cmake/cmake/-/issues/22427

$ cat /etc/synthetic.conf 
w	Users/gregorj/w

$ pwd                                        
/w/dev/test_void_p_size

$ stat ../../../Users/gregorj/src/cmake/Modules/CMakeCCompilerABI.c 
stat: ../../../Users/gregorj/src/cmake/Modules/CMakeCCompilerABI.c: stat: No such file or directory

$ stat /Users/gregorj/src/cmake/Modules/CMakeCCompilerABI.c 
16777220 20730161 -rw-r--r-- 1 gregorj staff 0 511 "Nov 21 16:13:29 2020" "Nov 21 16:13:29 2020" "Nov 21 16:13:29 2020" "Nov 21 16:13:29 2020" 4096 8 0 /Users/gregorj/src/cmake/Modules/CMakeCCompilerABI.c

If the pwd is “/w/dev/test_void_p_size” I’d expect that ../../.. is at the filesystem root (“/“). But listing files by  “ls ../../..” reveals files from “/Users/gregorj” instead. Strangely “cd ../../.. && ls” works as expected.

I tested on macOS 11.4 (build 20F71)

Thanks,
Gregor

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