[PATCH] linux-user: Adjust pgd_find_hole_fallback result with guest_loaddr

[Ivan A. Melnikov]

While pgd_find_hole_fallback returns the beginning of the
hole found, pgb_find_hole returns guest_base, which
is somewhat different as the binary qemu-user is loading
usually has non-zero load address.

Failing to take this into account leads to random crashes
if the hole is "just big enough", but not bigger:
in that case, important mappings (e.g. parts of qemu-user
itself) may be replaced with the binary it is loading
(e.g. the guest elf interpreter).

This patch also fixes the return type of pgd_find_hole_fallback:
it returns -1 if no hole is found, so a signed return type
should be used.

Downstream issue (in Russian): https://bugzilla.altlinux.org/39141
Signed-off-by: Ivan A. Melnikov <iv@altlinux.org>
---
 linux-user/elfload.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/linux-user/elfload.c b/linux-user/elfload.c
index bab4237e90..acd510532c 100644
--- a/linux-user/elfload.c
+++ b/linux-user/elfload.c
@@ -2205,9 +2205,11 @@ static void pgb_have_guest_base(const char *image_name, 
abi_ulong guest_loaddr,
  * /proc/self/map. It can potentially take a very long time as we can
  * only dumbly iterate up the host address space seeing if the
  * allocation would work.
+ *
+ * Returns the start addres of the hole found, or -1 if no hole found.
  */
-static uintptr_t pgd_find_hole_fallback(uintptr_t guest_size, uintptr_t brk,
-                                        long align, uintptr_t offset)
+static intptr_t pgd_find_hole_fallback(uintptr_t guest_size, uintptr_t brk,
+                                       long align, uintptr_t offset)
 {
     uintptr_t base;
 
@@ -2235,7 +2237,7 @@ static uintptr_t pgd_find_hole_fallback(uintptr_t 
guest_size, uintptr_t brk,
                 munmap((void *) align_start, guest_size);
                 if (MAP_FIXED_NOREPLACE != 0 ||
                     mmap_start == (void *) align_start) {
-                    return (uintptr_t) mmap_start + offset;
+                    return (intptr_t) mmap_start + offset;
                 }
             }
             base += qemu_host_page_size;
@@ -2259,7 +2261,8 @@ static uintptr_t pgb_find_hole(uintptr_t guest_loaddr, 
uintptr_t guest_size,
     brk = (uintptr_t)sbrk(0);
 
     if (!maps) {
-        return pgd_find_hole_fallback(guest_size, brk, align, offset);
+        ret = pgd_find_hole_fallback(guest_size, brk, align, offset);
+        return (ret > guest_loaddr) ? (ret - guest_loaddr) : -1;
     }
 
     /* The first hole is before the first map entry. */
-- 
2.29.2