Dear Christoph Hellwig,
( I guess you are the right person to ask this question ?)
The POSIX.2 specification of mprotect() says:
errorno should be set as ENOMEM if the addresses in the range [addr,
(addr + len)] are invalid for the address space of a process, or
specify one or more pages which are not mapped.
However, in the mprotect implementation (asmlinkage long
sys_mprotect(unsigned long start, size_t len, unsigned long prot)) in
linux/mm/mprotect.c file, if we call mprotect() with start as NULL and
len as 0, mprotect() returns 0 and it is not setting the errono.The
following code confirms this behaviour.
*********** mprotect check code ********************
#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <errno.h>
#include <sys/mman.h>
int main()
{
int fd, ret;
char *address;
errno = 0;
fd = open("./mmap_file", O_CREAT | O_RDWR, 766);
address = (char *) mmap(0, 100, PROT_READ, MAP_SHARED, fd, 0);
/* address argument is NULL and length argument is 0 */
if ((ret = mprotect(NULL, 0, PROT_READ)) == -1) {
printf("%s Error \n", strerror(errno));
printf("mprotect functionality is correct.\n");
} else {
printf("mprotect functionality needs to be verified \n");
exit(EXIT_FAILURE);
}
close(fd);
return 0;
}
*********** mprotect check code ********************
Is there a reason to return 0 when the len is 0 and start is NULL ? Is
it intentional ? If not, it should be fixed.
Cheers,
Maxin B. John
Bangalore, India
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