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

[Laurent Vivier <laurent@vivier.eu>]

Le 09/02/2016 16:57, Peter Maydell a écrit :
> 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>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>

> ---
>  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;
>          }
>