mprotect / pkey_mprotect - glibc

Split from the chapter glibc: Virtual Memory Allocation And Paging into the per-function layout; see the method page and the glibc ledger for coverage status.

Declaration and gates

#include <sys/mman.h>

int mprotect(void *addr, size_t len, int prot);   /* sys/mman.h:81 */

#define _GNU_SOURCE
#include <sys/mman.h>

int pkey_mprotect(void *addr, size_t len, int prot, int pkey);
#define PROT_NONE  0x0   /* bits/mman-linux.h:35 */
#define PROT_READ  0x1   /* bits/mman-linux.h:32 */
#define PROT_WRITE 0x2   /* bits/mman-linux.h:33 */
#define PROT_EXEC  0x4   /* bits/mman-linux.h:34 */
#define PROT_GROWSDOWN 0x01000000  /* bits/mman-linux.h:36 */
#define PROT_GROWSUP   0x02000000  /* bits/mman-linux.h:38 */

Semantics

mprotect changes the access protection of the pages covering [addr, addr+len-1); addr must be page-aligned (mprotect.2). prot is PROT_NONE or a mix of PROT_READ/PROT_WRITE/PROT_EXEC, plus the Linux-only PROT_GROWSDOWN/PROT_GROWSUP which extend the change across a downward/upward-growing mapping (stack). An access that violates the protection raises SIGSEGV for the process. The kernel may not support PROT_EXEC without PROT_READ on all architectures (header comment, bits/mman-linux.h lines 28-30).

pkey_mprotect (kernel 4.9, glibc 2.27) does the same but additionally assigns the protection key pkey – obtained from pkey_alloc – to the range; the key’s per-thread access rights (PKEY_DISABLE_ACCESS / PKEY_DISABLE_WRITE) are then enforced by the CPU on top of prot (mprotect.2; see the pkey family page). The raw syscall page is mprotect.

Gotchas and portability

• mprotect is the portable spelling (POSIX); pkey_mprotect is Linux-only and hardware-dependent (x86 PKU and a few others).
• mprotect on a range obtained from mmap of a read-only file cannot grant PROT_WRITE (EACCES); W^X policies may also forbid PROT_EXEC.
• musl 1.2.5: mprotect present (include/sys/mman.h line 119); no pkey_mprotect declaration or symbols anywhere in the tree.

Errors

EACCES (protection not grantable for that mapping), EFAULT (range not mapped / kernel can’t access), EINVAL (unaligned addr or bad prot; for pkey_mprotect, an unallocated pkey), ENOMEM (range not covering mapped pages in the caller’s space) (mprotect.2 ERRORS).

References

• man2: mprotect.2 (covers pkey_mprotect; pkey_mprotect.2 is a .so link)
• glibc manual: memory.texi (aligned/locking context), pkeys coverage in pkey_alloc.2
• headers: /usr/include/x86_64-linux-gnu/sys/mman.h, bits/mman-shared.h, bits/mman-linux.h
• symbols: nm -D --with-symbol-versions libc.so.6