CONFIG_MMAP_ALLOW_UNINITIALIZED: Allow mmapped anonymous memory to be uninitialized

General informations

The Linux kernel configuration item CONFIG_MMAP_ALLOW_UNINITIALIZED has multiple definitions:

Allow mmapped anonymous memory to be uninitialized found in mm/Kconfig

The configuration item CONFIG_MMAP_ALLOW_UNINITIALIZED:

• prompt: Allow mmapped anonymous memory to be uninitialized
• type: bool
• depends on: CONFIG_EXPERT && ! CONFIG_MMU
• defined in mm/Kconfig
• found in Linux kernels: 5.19, 6.0–6.8, 6.9-rc+HEAD

Help text

Normally, and according to the Linux spec, anonymous memory obtained from mmap() has its contents cleared before it is passed to userspace. Enabling this config option allows you to request that mmap() skip that if it is given an MAP_UNINITIALIZED flag, thus providing a huge performance boost. If this option is not enabled, then the flag will be ignored.

This is taken advantage of by uClibc's malloc(), and also by ELF-FDPIC binfmt's brk and stack allocator.

Because of the obvious security issues, this option should only be enabled on embedded devices where you control what is run in userspace. Since that isn't generally a problem on no-MMU systems, it is normally safe to say Y here.

See Documentation/admin-guide/mm/nommu-mmap.rst for more information.

Allow mmapped anonymous memory to be uninitialized found in init/Kconfig

The configuration item CONFIG_MMAP_ALLOW_UNINITIALIZED:

• prompt: Allow mmapped anonymous memory to be uninitialized
• type: bool
• depends on: CONFIG_EXPERT && ! CONFIG_MMU
• defined in init/Kconfig
• found in Linux kernels: 2.6.33–2.6.39, 3.0–3.19, 4.0–4.20, 5.0–5.18, 5.18+HEAD

Help text

Normally, and according to the Linux spec, anonymous memory obtained from mmap() has its contents cleared before it is passed to userspace. Enabling this config option allows you to request that mmap() skip that if it is given an MAP_UNINITIALIZED flag, thus providing a huge performance boost. If this option is not enabled, then the flag will be ignored.

This is taken advantage of by uClibc's malloc(), and also by ELF-FDPIC binfmt's brk and stack allocator.

Because of the obvious security issues, this option should only be enabled on embedded devices where you control what is run in userspace. Since that isn't generally a problem on no-MMU systems, it is normally safe to say Y here.

See Documentation/admin-guide/mm/nommu-mmap.rst for more information.

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