Re: [PATCH] cxgb4: fix undefined behavior in mem.c

[Bart Van Assche]

On Thu, 2019-02-28 at 15:38 -0700, Shaobo He wrote:
+AD4 In function +AGA-c4iw+AF8-dealloc+AF8-mw+AGA, variable mhp's value is printed after
+AD4 freed, which triggers undefined behavior according to this post:
+AD4 https://trust-in-soft.com/dangling-pointer-indeterminate/.
+AD4 
+AD4 This commit fixes it by swapping the order of +AGA-kfree+AGA and +AGA-pr+AF8-debug+AGA.
+AD4 
+AD4 Signed-off-by: Shaobo He +ADw-shaobo+AEA-cs.utah.edu+AD4
+AD4 ---
+AD4  drivers/infiniband/hw/cxgb4/mem.c +AHw 2 +--
+AD4  1 file changed, 1 insertion(+-), 1 deletion(-)
+AD4 
+AD4 diff --git a/drivers/infiniband/hw/cxgb4/mem.c b/drivers/infiniband/hw/cxgb4/mem.c
+AD4 index 7b76e6f..bb8e0bc 100644
+AD4 --- a/drivers/infiniband/hw/cxgb4/mem.c
+AD4 +-+-+- b/drivers/infiniband/hw/cxgb4/mem.c
+AD4 +AEAAQA -684,8 +-684,8 +AEAAQA int c4iw+AF8-dealloc+AF8-mw(struct ib+AF8-mw +ACo-mw)
+AD4  			  mhp-+AD4-wr+AF8-waitp)+ADs
+AD4  	kfree+AF8-skb(mhp-+AD4-dereg+AF8-skb)+ADs
+AD4  	c4iw+AF8-put+AF8-wr+AF8-wait(mhp-+AD4-wr+AF8-waitp)+ADs
+AD4 -	kfree(mhp)+ADs
+AD4  	pr+AF8-debug(+ACI-ib+AF8-mw +ACU-p mmid 0x+ACU-x ptr +ACU-p+AFw-n+ACI, mw, mmid, mhp)+ADs
+AD4 +-	kfree(mhp)+ADs
+AD4  	return 0+ADs
+AD4  +AH0

Please quote the relevant paragraphs from the C standard. All I have found
about free() in ISO/IEC 9899:2017 is the following:

Description
The free function causes the space pointed to by ptr to be deallocated, that
is, made available for further allocation. If ptr is a null pointer, no
action occurs. Otherwise, if the argument does not match a pointer earlier
returned by a memory management function, or if the space has been
deallocated by a call to free or realloc, the behavior is undefined.

That is not sufficient to claim that the above code triggers undefined
behavior.

Bart.