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bsd-user/mmap.c: Don't mmap fd == -1 independently from MAP_ANON flag

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Switch checks for !(flags & MAP_ANONYMOUS) with checks for fd != -1.
MAP_STACK and MAP_GUARD both require fd == -1 and don't require mapping
the fd either. Add analysis from Guy Yur detailing the different cases
for MAP_GUARD and MAP_STACK.

Signed-off-by: Guy Yur <guyyur@gmail.com>
[ partially merged before, finishing the job and documenting origin]
Signed-off-by: Warner Losh <imp@bsdimp.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Kyle Evans <kevans@FreeBSD.org>
This commit is contained in:
Warner Losh 2021-10-18 12:51:17 -06:00
parent 45b8765e8f
commit a6b2d06066
1 changed files with 25 additions and 5 deletions
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30
bsd-user/mmap.c
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@ -127,7 +127,27 @@ error:
return ret;
}
/* map an incomplete host page */
/*
* map an incomplete host page
*
* mmap_frag can be called with a valid fd, if flags doesn't contain one of
* MAP_ANON, MAP_STACK, MAP_GUARD. If we need to map a page in those cases, we
* pass fd == -1. However, if flags contains MAP_GUARD then MAP_ANON cannot be
* added.
*
* * If fd is valid (not -1) we want to map the pages with MAP_ANON.
* * If flags contains MAP_GUARD we don't want to add MAP_ANON because it
* will be rejected. See kern_mmap's enforcing of constraints for MAP_GUARD
* in sys/vm/vm_mmap.c.
* * If flags contains MAP_ANON it doesn't matter if we add it or not.
* * If flags contains MAP_STACK, mmap adds MAP_ANON when called so doesn't
* matter if we add it or not either. See enforcing of constraints for
* MAP_STACK in kern_mmap.
*
* Don't add MAP_ANON for the flags that use fd == -1 without specifying the
* flags directly, with the assumption that future flags that require fd == -1
* will also not require MAP_ANON.
*/
static int mmap_frag(abi_ulong real_start,
abi_ulong start, abi_ulong end,
int prot, int flags, int fd, abi_ulong offset)
@ -147,9 +167,9 @@ static int mmap_frag(abi_ulong real_start,
}
if (prot1 == 0) {
/* no page was there, so we allocate one */
/* no page was there, so we allocate one. See also above. */
void *p = mmap(host_start, qemu_host_page_size, prot,
flags | MAP_ANON, -1, 0);
flags | ((fd != -1) ? MAP_ANON : 0), -1, 0);
if (p == MAP_FAILED)
return -1;
prot1 = prot;
@ -157,7 +177,7 @@ static int mmap_frag(abi_ulong real_start,
prot1 &= PAGE_BITS;
prot_new = prot | prot1;
if (!(flags & MAP_ANON)) {
if (fd != -1) {
/* msync() won't work here, so we return an error if write is
possible while it is a shared mapping */
if ((flags & TARGET_BSD_MAP_FLAGMASK) == MAP_SHARED &&
@ -565,7 +585,7 @@ abi_long target_mmap(abi_ulong start, abi_ulong len, int prot,
* worst case: we cannot map the file because the offset is not
* aligned, so we read it
*/
if (!(flags & MAP_ANON) &&
if (fd != -1 &&
(offset & ~qemu_host_page_mask) != (start & ~qemu_host_page_mask)) {
/*
* msync() won't work here, so we return an error if write is