[Qemu-devel] [PATCH] linux-user: Don't assert if guest tries shmdt(0)

[Peter Maydell <peter.maydell@linaro.org>]

Our implementation of shmat() and shmdt() for linux-user was
using "zero guest address" as its marker for "entry in the
shm_regions[] array is not in use". This meant that if the
guest did a shmdt(0) we would match on an unused array entry
and call page_set_flags() with both start and end addresses zero,
which causes an assertion failure.

Use an explicit in_use flag to manage the shm_regions[] array,
so that we avoid this problem.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reported-by: Pavel Shamis <pasharesearch@gmail.com>
---
 linux-user/syscall.c | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/linux-user/syscall.c b/linux-user/syscall.c
index 54ce14a..f46abf7 100644
--- a/linux-user/syscall.c
+++ b/linux-user/syscall.c
@@ -2598,8 +2598,9 @@ static abi_long do_socketcall(int num, abi_ulong vptr)
 #define N_SHM_REGIONS	32
 
 static struct shm_region {
-    abi_ulong	start;
-    abi_ulong	size;
+    abi_ulong start;
+    abi_ulong size;
+    bool in_use;
 } shm_regions[N_SHM_REGIONS];
 
 struct target_semid_ds
@@ -3291,7 +3292,8 @@ static inline abi_ulong do_shmat(int shmid, abi_ulong shmaddr, int shmflg)
                    ((shmflg & SHM_RDONLY)? 0 : PAGE_WRITE));
 
     for (i = 0; i < N_SHM_REGIONS; i++) {
-        if (shm_regions[i].start == 0) {
+        if (!shm_regions[i].in_use) {
+            shm_regions[i].in_use = true;
             shm_regions[i].start = raddr;
             shm_regions[i].size = shm_info.shm_segsz;
             break;
@@ -3308,8 +3310,8 @@ static inline abi_long do_shmdt(abi_ulong shmaddr)
     int i;
 
     for (i = 0; i < N_SHM_REGIONS; ++i) {
-        if (shm_regions[i].start == shmaddr) {
-            shm_regions[i].start = 0;
+        if (shm_regions[i].in_use && shm_regions[i].start == shmaddr) {
+            shm_regions[i].in_use = false;
             page_set_flags(shmaddr, shmaddr + shm_regions[i].size, 0);
             break;
         }
-- 
1.9.1