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[Qemu-devel] [PATCH 1/3] linux-user: Don't use MAP_FIXED in do_brk()
From: |
Peter Maydell |
Subject: |
[Qemu-devel] [PATCH 1/3] linux-user: Don't use MAP_FIXED in do_brk() |
Date: |
Mon, 18 Apr 2011 16:34:24 +0100 |
Since mmap() with MAP_FIXED will map over the top of existing mappings,
it's a bad idea to use it to implement brk(), because brk() with a
large size is likely to overwrite important things like qemu itself
or the host libc. So we drop MAP_FIXED and handle "mapped but at
different address" as an error case instead.
Signed-off-by: Peter Maydell <address@hidden>
---
linux-user/syscall.c | 29 ++++++++++++++++++++---------
1 files changed, 20 insertions(+), 9 deletions(-)
diff --git a/linux-user/syscall.c b/linux-user/syscall.c
index bb0999d..e68c5e0 100644
--- a/linux-user/syscall.c
+++ b/linux-user/syscall.c
@@ -733,23 +733,34 @@ abi_long do_brk(abi_ulong new_brk)
return target_brk;
}
- /* We need to allocate more memory after the brk... */
+ /* We need to allocate more memory after the brk... Note that
+ * we don't use MAP_FIXED because that will map over the top of
+ * any existing mapping (like the one with the host libc or qemu
+ * itself); instead we treat "mapped but at wrong address" as
+ * a failure and unmap again.
+ */
new_alloc_size = HOST_PAGE_ALIGN(new_brk - brk_page + 1);
mapped_addr = get_errno(target_mmap(brk_page, new_alloc_size,
PROT_READ|PROT_WRITE,
- MAP_ANON|MAP_FIXED|MAP_PRIVATE, 0, 0));
+ MAP_ANON|MAP_PRIVATE, 0, 0));
+
+ if (mapped_addr == brk_page) {
+ target_brk = new_brk;
+ return target_brk;
+ } else if (mapped_addr != -1) {
+ /* Mapped but at wrong address, meaning there wasn't actually
+ * enough space for this brk.
+ */
+ target_munmap(mapped_addr, new_alloc_size);
+ mapped_addr = -1;
+ }
#if defined(TARGET_ALPHA)
/* We (partially) emulate OSF/1 on Alpha, which requires we
return a proper errno, not an unchanged brk value. */
- if (is_error(mapped_addr)) {
- return -TARGET_ENOMEM;
- }
+ return -TARGET_ENOMEM;
#endif
-
- if (!is_error(mapped_addr)) {
- target_brk = new_brk;
- }
+ /* For everything else, return the previous break. */
return target_brk;
}
--
1.7.1