Re: [PATCH] util: Remove redundant checks in the openpty()

[AlexChen]

On 2020/10/31 23:21, Peter Maydell wrote:
> On Sat, 31 Oct 2020 at 11:04, AlexChen <alex.chen@huawei.com> wrote:
>>
>> As we can see from the following function call stack, the amaster and the 
>> aslave
>> cannot be NULL: char_pty_open() -> qemu_openpty_raw() -> openpty().
>> In addition, the amaster and the aslave has been dereferenced at the 
>> beginning
>> of the openpty(). So the checks on amaster and aslave in the openpty() are 
>> redundant.
>>
>> Reported-by: Euler Robot <euler.robot@huawei.com>
>> Signed-off-by: Alex Chen <alex.chen@huawei.com>
> 
> This function is trying to match the BSD/glibc openpty()
> function, so the thing to check here is not QEMU's specific
> current usage but the API specification for openpty():
> https://www.gnu.org/software/libc/manual/html_node/Pseudo_002dTerminal-Pairs.html
> https://www.freebsd.org/cgi/man.cgi?query=openpty
> 
> The spec says that name, termp and winp can all be
> NULL, but it doesn't say this for amaster and aslave,
> so indeed the change in this patch is the correct one.
> 
>> ---
>>  util/qemu-openpty.c | 7 +++----
>>  1 file changed, 3 insertions(+), 4 deletions(-)
>>
>> diff --git a/util/qemu-openpty.c b/util/qemu-openpty.c
>> index eb17f5b0bc..427f43a769 100644
>> --- a/util/qemu-openpty.c
>> +++ b/util/qemu-openpty.c
>> @@ -80,10 +80,9 @@ static int openpty(int *amaster, int *aslave, char *name,
>>              (termp != NULL && tcgetattr(sfd, termp) < 0))
>>                  goto err;
>>
>> -        if (amaster)
>> -                *amaster = mfd;
>> -        if (aslave)
>> -                *aslave = sfd;
>> +        *amaster = mfd;
>> +        *aslave = sfd;
>> +
>>          if (winp)
>>                  ioctl(sfd, TIOCSWINSZ, winp);
> 
> Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
> 
> though you might like to mention in the commit message that
> the openpty() API doesn't allow NULL amaster or aslave
> arguments.
>

Thanks for your review, I will add this description to my commit message in my 
patch V2.
In addition, since the amaster and the aslave are not allow to be NULL,
do we need to check that the amaster and the aslave are NULL in the beginning 
of the openpty()?
such as this modification:

diff --git a/util/qemu-openpty.c b/util/qemu-openpty.c
index eb17f5b0bc..1aadd39395 100644
--- a/util/qemu-openpty.c
+++ b/util/qemu-openpty.c
@@ -61,6 +61,9 @@ static int openpty(int *amaster, int *aslave, char *name,
         const char *slave;
         int mfd = -1, sfd = -1;

+        if (!amaster || !aslave)
+                goto err;
+
         *amaster = *aslave = -1;

         mfd = open("/dev/ptmx", O_RDWR | O_NOCTTY);
@@ -80,10 +83,9 @@ static int openpty(int *amaster, int *aslave, char *name,
             (termp != NULL && tcgetattr(sfd, termp) < 0))
                 goto err;

-        if (amaster)
-                *amaster = mfd;
-        if (aslave)
-                *aslave = sfd;
+        *amaster = mfd;
+        *aslave = sfd;
+
         if (winp)
                 ioctl(sfd, TIOCSWINSZ, winp);

@@ -92,7 +94,8 @@ static int openpty(int *amaster, int *aslave, char *name,
 err:
         if (sfd != -1)
                 close(sfd);
-        close(mfd);
+        if (mfd != -1)
+                close(mfd);
         return -1;
 }
 #endif
-- 
2.19.1

Thanks,
Alex