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linux-user: Add emulation for MADV_WIPEONFORK and MADV_KEEPONFORK in madvise() 

Both parameters have a different value on the parisc platform, so first
translate the target value into a host value for usage in the native
madvise() syscall.

Those parameters are often used by security sensitive applications (e.g.
tor browser, boringssl, ...) which expect the call to return a proper
return code on failure, so return -EINVAL if qemu fails to forward the
syscall to the host OS.

While touching this code, enhance the comments about MADV_DONTNEED.

Tested with testcase of tor browser when running hppa-linux guest on
x86-64 host.

Signed-off-by: Helge Deller <deller@gmx.de>
Acked-by: Ilya Leoshkevich <iii@linux.ibm.com>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-Id: <Y5iwTaydU7i66K/i@p100>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>
This commit is contained in:
Helge Deller 2022-12-13 18:03:09 +01:00 committed by Laurent Vivier
parent ab6c497e7e
commit 4530deb1fe
1 changed files with 43 additions and 13 deletions
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56
linux-user/mmap.c
View File

@ -857,7 +857,7 @@ abi_long target_mremap(abi_ulong old_addr, abi_ulong old_size,
return new_addr;
}
static bool can_passthrough_madv_dontneed(abi_ulong start, abi_ulong end)
static bool can_passthrough_madvise(abi_ulong start, abi_ulong end)
{
ulong addr;
@ -901,23 +901,53 @@ abi_long target_madvise(abi_ulong start, abi_ulong len_in, int advice)
return -TARGET_EINVAL;
}
/* Translate for some architectures which have different MADV_xxx values */
switch (advice) {
case TARGET_MADV_DONTNEED: /* alpha */
advice = MADV_DONTNEED;
break;
case TARGET_MADV_WIPEONFORK: /* parisc */
advice = MADV_WIPEONFORK;
break;
case TARGET_MADV_KEEPONFORK: /* parisc */
advice = MADV_KEEPONFORK;
break;
/* we do not care about the other MADV_xxx values yet */
}
/*
* A straight passthrough may not be safe because qemu sometimes turns
* private file-backed mappings into anonymous mappings.
* Most advice values are hints, so ignoring and returning success is ok.
*
* This is a hint, so ignoring and returning success is ok.
* However, some advice values such as MADV_DONTNEED, MADV_WIPEONFORK and
* MADV_KEEPONFORK are not hints and need to be emulated.
*
* This breaks MADV_DONTNEED, completely implementing which is quite
* complicated. However, there is one low-hanging fruit: mappings that are
* known to have the same semantics in the host and the guest. In this case
* passthrough is safe, so do it.
* A straight passthrough for those may not be safe because qemu sometimes
* turns private file-backed mappings into anonymous mappings.
* can_passthrough_madvise() helps to check if a passthrough is possible by
* comparing mappings that are known to have the same semantics in the host
* and the guest. In this case passthrough is safe.
*
* We pass through MADV_WIPEONFORK and MADV_KEEPONFORK if possible and
* return failure if not.
*
* MADV_DONTNEED is passed through as well, if possible.
* If passthrough isn't possible, we nevertheless (wrongly!) return
* success, which is broken but some userspace programs fail to work
* otherwise. Completely implementing such emulation is quite complicated
* though.
*/
mmap_lock();
if (advice == TARGET_MADV_DONTNEED &&
can_passthrough_madv_dontneed(start, end)) {
ret = get_errno(madvise(g2h_untagged(start), len, MADV_DONTNEED));
if (ret == 0) {
page_reset_target_data(start, start + len);
switch (advice) {
case MADV_WIPEONFORK:
case MADV_KEEPONFORK:
ret = -EINVAL;
/* fall through */
case MADV_DONTNEED:
if (can_passthrough_madvise(start, end)) {
ret = get_errno(madvise(g2h_untagged(start), len, advice));
if ((advice == MADV_DONTNEED) && (ret == 0)) {
page_reset_target_data(start, start + len);
}
}
}
mmap_unlock();