fsuidgid_has_mapping(9)

💅 fsuidgid_has_mapping

okay so bestie, this is a page from the Kernel API Manual — section 9, kernel-land, very much main character energy for people who write filesystems ✨. The function is literally called fsuidgid_has_mapping, and yes, that name is a mouthful, but we're gonna get through it together 🫶.

✨ NAME ✨

fsuidgid_has_mapping — check whether caller's fsuid/fsgid is mapped 💖

🌸 SYNOPSIS 🌸

bool fsuidgid_has_mapping (struct super_block *sb , struct mnt_idmap
*idmap );

like, it returns a bool, takes two pointers, that's the whole vibe check 🔥

🎀 ARGUMENTS 🎀

• sb — the superblock we want a mapping in, bestie. that's the one.
• idmap — the idmap of the relevant mount. the mount's whole identity, honestly 👑

🦋 DESCRIPTION 🦋

okay so here's the tea ☕: this function checks whether the caller's fsuid and fsgid have a valid mapping in the s_user_ns of the superblock sb. that's it, that's the job 💅

but wait — there's a little twist, and it's giving conditional logic ✨: if the caller is on an idmapped mount, then map the caller's fsuid and fsgid according to the idmap first. so like, idmap translation happens before the check, not after. periodt 🌈

⭐ RETURN ⭐

• true if fsuid and fsgid is mapped
• false if not

no in-between, no "it depends", just a clean yes or no, we love a decisive icon 😌💖

👑 SEE ALSO 👑

Kernel file ./include/linux/fs.h — and then a whole friend group of related things, ngl this list is long but every single one of them ate 💀:

• enum positive_aop_returns(9)
• struct address_space(9)
• struct file_ra_state(9)
• struct file(9)
• i_uid_into_vfsuid(9)
• i_uid_needs_update(9)
• i_uid_update(9)
• i_gid_into_vfsgid(9)
• i_gid_needs_update(9)
• i_gid_update(9)
• inode_fsuid_set(9)
• inode_fsgid_set(9)
• inode_set_ctime(9)
• file_write_started(9)
• file_write_not_started(9)
• struct renamedata(9)
• is_mgtime(9)
• iput_if_not_last(9)
• is_idmapped_mnt(9)
• file_start_write(9)
• file_end_write(9)
• kiocb_start_write(9)
• kiocb_end_write(9)
• name_is_dot_dotdot(9)
• name_contains_dotdot(9)
• inode_dio_begin(9)
• inode_dio_end(9)
• generic_ci_validate_strict_name(9)

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and that's the page, bestie 🧚💜 — brought to you from the ./include/linux corner of the kernel, dated October 2026, for fsuidgid_has_mapping(9) in the Kernel API Manual. go forth and check those mappings, ily, bye 🎀✨