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Does the suid bit have any special meaning for device files in Linux ?

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find /dev -perm /ugo+s gives nothing on my machine (nor a few others I checked); does your machine have set id/sticky entries in /dev ? – derobert Sep 13 '12 at 15:45

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I believe it is not. This bit is only used on executable files. It's defined in Linux kernel headers as S_ISUID. If you grep kernel sources for this constant, you will find that it is only used in:

  • should_remove_suid function, which is used on FS operations that should remove SUID/SGID bit,
  • prepare_binprm function in fs/exec.c which is used when prepairing executable file to set EUID on exec,
  • pid_revalidate function in fs/proc/base.c which is used to populate procfs,
  • notify_change function in fs/attr.c which is used when changing file attributes,
  • is_sxid function in include/linux/fs.h which is only used by XFS and GFS specific code and notify_change function,
  • in filesystem specific code (of course)

So it seems to me that this bit is only used (from userspace perspective) when executing files. At least on Linux.

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Don't forget some of these bits serve a purpose for DIRECTories too. – mdpc Sep 13 '12 at 22:14
@mdpc Directories are also files. – Chris Down Sep 14 '12 at 0:34
@mdpc: SUID bit on directory has a meaning at least on ext2/3 filesystem. Some other filesystems may implement some mechanisms similar to SGID bit on directories. But that's filesystem dependent so it only involves filesystem operations. – Krzysztof Adamski Sep 14 '12 at 6:00
Thank you for detailed explanation and answer – hostmaster Sep 14 '12 at 6:51

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