Most flags to madvise() are just hints, so typically ignoring the
syscall and returning okay is fine. However applications exist that do
rely on MADV_DONTNEED behavior to guarantee that upon subsequent access
the mapping is refreshed from the backing file or zero for anonymous
mappings.
Signed-off-by: Simon Hausmann <simon.hausm...@qt.io>
---
linux-user/mmap.c | 21 +++++++++++++++++++++
linux-user/qemu.h | 1 +
linux-user/syscall.c | 6 +-----
3 files changed, 23 insertions(+), 5 deletions(-)
diff --git a/linux-user/mmap.c b/linux-user/mmap.c
index 41e0983ce8..6b308d4e13 100644
--- a/linux-user/mmap.c
+++ b/linux-user/mmap.c
@@ -762,3 +762,24 @@ abi_long target_mremap(abi_ulong old_addr, abi_ulong
old_size,
mmap_unlock();
return new_addr;
}
+
+int target_madvise(abi_ulong start, abi_ulong len, int flags)
+{
+ len = HOST_PAGE_ALIGN(len);
+ start &= qemu_host_page_mask;
+
+ if (!guest_range_valid(start, len)) {
+ errno = TARGET_EINVAL;
+ return -1;
+ }
+
+ /* A straight passthrough may not be safe because qemu sometimes
+ turns private file-backed mappings into anonymous mappings.
+ Most flags are hints, except for MADV_DONTNEED that applications
+ may rely on to zero out pages, so we pass that through.
+ Otherwise returning success is ok. */
+ if (flags & MADV_DONTNEED) {
+ return madvise(g2h(start), len, MADV_DONTNEED);
+ }
+ return 0;
+}
diff --git a/linux-user/qemu.h b/linux-user/qemu.h
index b4959e41c6..4b68019904 100644
--- a/linux-user/qemu.h
+++ b/linux-user/qemu.h
@@ -437,6 +437,7 @@ int target_munmap(abi_ulong start, abi_ulong len);
abi_long target_mremap(abi_ulong old_addr, abi_ulong old_size,
abi_ulong new_size, unsigned long flags,
abi_ulong new_addr);
+int target_madvise(abi_ulong start, abi_ulong len, int flags);
extern unsigned long last_brk;
extern abi_ulong mmap_next_start;
abi_ulong mmap_find_vma(abi_ulong, abi_ulong);
diff --git a/linux-user/syscall.c b/linux-user/syscall.c
index 202aa777ad..023874ac8c 100644
--- a/linux-user/syscall.c
+++ b/linux-user/syscall.c
@@ -11874,11 +11874,7 @@ abi_long do_syscall(void *cpu_env, int num, abi_long
arg1,
#ifdef TARGET_NR_madvise
case TARGET_NR_madvise:
- /* A straight passthrough may not be safe because qemu sometimes
- turns private file-backed mappings into anonymous mappings.
- This will break MADV_DONTNEED.
- This is a hint, so ignoring and returning success is ok. */
- ret = get_errno(0);
+ ret = get_errno(target_madvise(arg1, arg2, arg3));
break;
#endif
#if TARGET_ABI_BITS == 32
--
2.17.1
