mprotect(2)

1MPROTECT(2)                Linux Programmer's Manual               MPROTECT(2)
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NAME

6       mprotect - control allowable accesses to a region of memory
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SYNOPSIS

9       #include <sys/mman.h>
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11       int mprotect(const void *addr, size_t len, int prot);
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DESCRIPTION

14       The function mprotect() specifies the desired protection for the memory
15       page(s) containing part or all of the interval  [addr,addr+len-1].   If
16       an  access  is  disallowed  by  the  protection  given  it, the program
17       receives a SIGSEGV.
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19       prot is a bitwise-or of the following values:
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21       PROT_NONE  The memory cannot be accessed at all.
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23       PROT_READ  The memory can be read.
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25       PROT_WRITE The memory can be written to.
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27       PROT_EXEC  The memory can contain executing code.
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29       The new protection replaces any existing protection.  For  example,  if
30       the memory had previously been marked PROT_READ, and mprotect() is then
31       called with prot PROT_WRITE, it will no longer be readable.
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RETURN VALUE

34       On success, mprotect() returns zero.  On error,  -1  is  returned,  and
35       errno is set appropriately.
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ERRORS

38       EACCES The  memory cannot be given the specified access.  This can hap‐
39              pen, for example, if you mmap(2) a file to which you have  read-
40              only access, then ask mprotect() to mark it PROT_WRITE.
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42       EFAULT The memory cannot be accessed.
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44       EINVAL addr is not a valid pointer, or not a multiple of PAGESIZE.
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46       ENOMEM Internal   kernel   structures  could  not  be  allocated.   Or:
47              addresses  in the range [addr, addr+len]  are  invalid  for  the
48              address  space of the process, or specify one or more pages that
49              are not mapped.
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EXAMPLE

52       #include <stdio.h>
53       #include <stdlib.h>
54       #include <errno.h>
55       #include <sys/mman.h>
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57       #include <limits.h>    /* for PAGESIZE */
58       #ifndef PAGESIZE
59       #define PAGESIZE 4096
60       #endif
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62       int
63       main(void)
64       {
65           char *p;
66           char c;
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68           /* Allocate a buffer; it will have the default
69              protection of PROT_READ|PROT_WRITE. */
70           p = malloc(1024+PAGESIZE-1);
71           if (!p) {
72               perror("Couldn't malloc(1024)");
73               exit(errno);
74           }
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76           /* Align to a multiple of PAGESIZE, assumed to be a power of two */
77           p = (char *)(((int) p + PAGESIZE-1) & ~(PAGESIZE-1));
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79           c = p[666];         /* Read; ok */
80           p[666] = 42;        /* Write; ok */
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82           /* Mark the buffer read-only. */
83           if (mprotect(p, 1024, PROT_READ)) {
84               perror("Couldn't mprotect");
85               exit(errno);
86           }
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88           c = p[666];         /* Read; ok */
89           p[666] = 42;        /* Write; program dies on SIGSEGV */
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91           exit(0);
92       }
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CONFORMING TO

95       SVr4, POSIX.1-2001.  POSIX says that mprotect() can  be  used  only  on
96       regions of memory obtained from mmap(2).
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NOTES

99       On  Linux  it  is  always  legal to call mprotect() on any address in a
100       process' address space (except for the kernel vsyscall area).  In  par‐
101       ticular it can be used to change existing code mappings to be writable.
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103       Whether  PROT_EXEC has any effect different from PROT_READ is architec‐
104       ture and kernel version dependent.
105