Manpage of MPROTECT


Section: Linux Programmer's Manual (2)
Updated: 2017-09-15


mprotect, pkey_mprotect - set protection on a region of memory  


#include <sys/mman.h>int mprotect(void *addr, size_t len, int prot);int pkey_mprotect(void *addr, size_t len, int prot, int pkey);


mprotect() changes the access protections for the calling process's memory pages containing any part of the address range in the interval [addraddr+len-1]. addrmust be aligned to a page boundary.

If the calling process tries to access memory in a manner that violates the protections, then the kernel generates a SIGSEGVsignal for the process.

protis a combination of the following access flags: PROT_NONEor a bitwise-or of the other values in the following list:

The memory cannot be accessed at all.
The memory can be read.
The memory can be modified.
The memory can be executed.
PROT_SEM (since Linux 2.5.7)
The memory can be used for atomic operations. This flag was introduced as part of the futex(2) implementation (in order to guarantee the ability to perform atomic operations required by commands such as FUTEX_WAIT), but is not currently used in on any architecture.
PROT_SAO (since Linux 2.6.26)
The memory should have strong access ordering. This feature is specific to the PowerPC architecture (version 2.06 of the architecture specification adds the SAO CPU feature, and it is available on POWER 7 or PowerPC A2, for example).

Additionally (since Linux 2.6.0), protcan have one of the following flags set:

PROT_GROWSUPApply the protection mode up to the end of a mapping that grows upwards. (Such mappings are created for the stack area on architectures---for example, HP-PARISC---that have an upwardly growing stack.)
Apply the protection mode down to the beginning of a mapping that grows downward (which should be a stack segment or a segment mapped with the MAP_GROWSDOWNflag set).

Like mprotect(), pkey_mprotect() changes the protection on the pages specified by addrand len. The pkeyargument specifies the protection key (see pkeys(7)) to assign to the memory. The protection key must be allocated with pkey_alloc(2) before it is passed to pkey_mprotect(). For an example of the use of this system call, see pkeys(7).  


On success, mprotect() and pkey_mprotect() return zero. On error, these system calls return -1, and errnois set appropriately.  


The memory cannot be given the specified access. This can happen, for example, if you mmap(2) a file to which you have read-only access, then ask mprotect() to mark it PROT_WRITE.
addr is not a valid pointer, or not a multiple of the system page size.
(pkey_mprotect()) pkey has not been allocated with pkey_alloc(2)
Both PROT_GROWSUPand PROT_GROWSDOWNwere specified in prot.
Invalid flags specified in prot.
(PowerPC architecture) PROT_SAOwas specified in prot, but SAO hardware feature is not available.
Internal kernel structures could not be allocated.
Addresses in the range [addr, addr+len-1] are invalid for the address space of the process, or specify one or more pages that are not mapped. (Before kernel 2.4.19, the error EFAULTwas incorrectly produced for these cases.)
Changing the protection of a memory region would result in the total number of mappings with distinct attributes (e.g., read versus read/write protection) exceeding the allowed maximum. (For example, making the protection of a range PROT_READin the middle of a region currently protected as PROT_READ|PROT_WRITEwould result in three mappings: two read/write mappings at each end and a read-only mapping in the middle.)


pkey_mprotect() first appeared in Linux 4.9. Glibc support is not yet available.  


mprotect(): POSIX.1-2001, POSIX.1-2008, SVr4. POSIX says that the behavior of mprotect() is unspecified if it is applied to a region of memory that was not obtained via mmap(2).

pkey_mprotect() is a nonportable Linux extension.  


On Linux, it is always permissible to call mprotect() on any address in a process's address space (except for the kernel vsyscall area). In particular, it can be used to change existing code mappings to be writable.

Whether PROT_EXEChas any effect different from PROT_READdepends on processor architecture, kernel version, and process state. If READ_IMPLIES_EXECis set in the process's personality flags (see personality(2)), specifying PROT_READwill implicitly add PROT_EXEC.

On some hardware architectures (e.g., i386), PROT_WRITEimplies PROT_READ.

POSIX.1 says that an implementation may permit access other than that specified in prot, but at a minimum can allow write access only if PROT_WRITEhas been set, and must not allow any access if PROT_NONEhas been set.

Applications should be careful when mixing use of mprotect() and pkey_mprotect(). On x86, when mprotect() is used with protset to PROT_EXECa pkey is may be allocated and set on the memory implicitly by the kernel, but only when the pkey was 0 previously.

On systems that do not support protection keys in hardware, pkey_mprotect() may still be used, but pkeymust be set to 0. When called this way, the operation of pkey_mprotect() is equivalent to mprotect().  


The program below demonstrates the use of mprotect(). The program allocates four pages of memory, makes the third of these pages read-only, and then executes a loop that walks upward through the allocated region modifying bytes.

An example of what we might see when running the program is the following:

$ ./a.outStart of region: 0x804c000 Got SIGSEGV at address: 0x804e000  

Program source

#include <unistd.h> #include <signal.h> #include <stdio.h> #include <malloc.h> #include <stdlib.h> #include <errno.h> #include <sys/mman.h>

#define handle_error(msg) \
    do { perror(msg); exit(EXIT_FAILURE); } while (0)

static char *buffer;

static void handler(int sig, siginfo_t *si, void *unused) {
    /* Note: calling printf() from a signal handler is not safe
       (and should not be done in production programs), since
       printf() is not async-signal-safe; see signal-safety(7).
       Nevertheless, we use printf() here as a simple way of
       showing that the handler was called. */

    printf("Got SIGSEGV at address: 0x%lx\n",
            (long) si->si_addr);
    exit(EXIT_FAILURE); }

int main(int argc, char *argv[]) {
    char *p;
    int pagesize;
    struct sigaction sa;

    sa.sa_flags = SA_SIGINFO;
    sa.sa_sigaction = handler;
    if (sigaction(SIGSEGV, &sa, NULL) == -1)

    pagesize = sysconf(_SC_PAGE_SIZE);
    if (pagesize == -1)

    /* Allocate a buffer aligned on a page boundary;
       initial protection is PROT_READ | PROT_WRITE */

    buffer = memalign(pagesize, 4 * pagesize);
    if (buffer == NULL)

    printf("Start of region:        0x%lx\n", (long) buffer);

    if (mprotect(buffer + pagesize * 2, pagesize,
                PROT_READ) == -1)

    for (p = buffer ; ; )
        *(p++) = aqaaq;

    printf("Loop completed\n");     /* Should never happen */
    exit(EXIT_SUCCESS); }  


mmap(2), sysconf(3), pkeys(7)



Program source

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