On Thu, Feb 11, 2010 at 08:24:52PM -0500, Ted Unangst wrote:
> The man page is likely missing the word "no", as that makes a lot more
> sense than the current wording meaning positive action.
>
well, can you fix that then, please?
jmc
> 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
