The man page is likely missing the word "no", as that makes a lot more
sense than the current wording meaning positive action.
2010/2/11 Vadim Zhukov <persg...@gmail.com>:
> Hello all.
>
> mprotect(2) says: "If len is 0, then action will be taken on the page
> that contains addr". The reality says it's not so:
>
> $ cat ~/cvs/mprotect_test.c
>
> #include <sys/types.h>
> #include <sys/param.h>
> #include <sys/mman.h>
>
> #include <stdio.h>
> #include <unistd.h>
>
> void
> mymemset(char *p, int c, size_t len) {
> size_t i;
> char cc;
>
> cc = (char)(c & 0xFF);
> for (i = 0; i < len; i++) {
> fputc('.', stderr);
> p[i] = cc;
> }
> }
>
> int
> main(int argc, char **argv) {
> long pgsz;
> char *p;
>
> pgsz = sysconf(_SC_PAGESIZE);
> if ((p = (char*)mmap(NULL, pgsz * 3, PROT_NONE, MAP_ANON, -1, 0)) ==
NULL)
> err(1, "mmap");
> if (mprotect(p + pgsz, 0, PROT_READ|PROT_WRITE) == -1)
> err(1, mprotect);
> fputs("writing to page 1: ", stderr);
> mymemset(p + pgsz, 0xCC, pgsz);
> fputs("OK\nwriting to page 0: ", stderr);
> mymemset(p, 0xCC, pgsz);
> fputs("OK (WTF?!)\nwriting to page 2: ", stderr);
> mymemset(p + (pgsz * 2), 0xCC, pgsz);
> fputs("OK (WTF?!)\n", stderr);
>
> return 1;
> }
> $ cc -O0 -ggdb -o mprotect_test mprotect_test.c
> $ ./mprotect_test
> writing to page 1: .Segmentation fault (core dumped)
> $
>
> I checked the SUS, and it is not clear: "The mprotect() function shall
> change the access protections to be that specified by prot for those
> whole pages containing any part of the address space of the process
> starting at address addr and continuing for len bytes."
>
> So either sys_mprotect(), uvm_map_protect() or man page is not correct.
> As I'm totally newbie in UVM, please, do not bite me for the patch
> proposed below. At least, very much. :)
>
> --
> Best wishes,
> Vadim Zhukov
>
> A: Because it messes up the order in which people normally read text.
> Q: Why is top-posting such a bad thing?
> A: Top-posting.
> Q: What is the most annoying thing in e-mail?
>
>
> @@ -778,6 +778,8 @@ sys_mprotect(struct proc *p, void *v, re
>
> if ((prot & VM_PROT_ALL) != prot)
> return (EINVAL);
> + if (size == 0)
> + size = PAGE_SIZE;
>
> /*
> * align the address to a page boundary, and adjust the size
accordingly
