On Tue, 21 May 2019, Gen Zhang wrote: > On Mon, May 20, 2019 at 11:26:20PM -0400, Nicolas Pitre wrote: > > On Tue, 21 May 2019, Gen Zhang wrote: > > > > > On Mon, May 20, 2019 at 10:55:40PM -0400, Nicolas Pitre wrote: > > > > On Tue, 21 May 2019, Gen Zhang wrote: > > > > > > > > > In function con_init(), the pointer variable vc_cons[currcons].d, vc > > > > > and > > > > > vc->vc_screenbuf is allocated a memory space via kzalloc(). And they > > > > > are > > > > > used in the following codes. > > > > > However, when there is a memory allocation error, kzalloc() can fail. > > > > > Thus null pointer (vc_cons[currcons].d, vc and vc->vc_screenbuf) > > > > > dereference may happen. And it will cause the kernel to crash. > > > > > Therefore, > > > > > we should check return value and handle the error. > > > > > Further,the loop condition MIN_NR_CONSOLES is defined as 1 in > > > > > include/uapi/linux/vt.h. So there is no need to unwind the loop. > > > > > > > > But what if someone changes that define? It won't be obvious that some > > > > code did rely on it to be defined to 1. > > > I re-examine the source code. MIN_NR_CONSOLES is only defined once and > > > no other changes to it. > > > > Yes, that is true today. But if someone changes that in the future, how > > will that person know that you relied on it to be 1 for not needing to > > unwind the loop? > > > > > > Nicolas > Hi Nicolas, > Thanks for your explaination! And I got your point. And is this way > proper?
Not quite. > err_vc_screenbuf: > kfree(vc); > for (currcons = 0; currcons < MIN_NR_CONSOLES; currcons++) > vc_cons[currcons].d = NULL; > return -ENOMEM; > err_vc: > console_unlock(); > return -ENOMEM; Now imagine that MIN_NR_CONSOLES is defined to 10 instead of 1. What happens with allocated memory if the err_vc condition is met on the 5th loop? If err_vc_screenbuf condition is encountered on the 5th loop (curcons = 4), what is the value of vc_cons[4].d? Isn't it the same as vc that you just freed? Nicolas
