RE: [GIT PULL] percpu fix for v5.9-rc6

[David Laight]

From: Arvind Sankar
> Sent: 18 September 2020 23:40
..
> Ouch, offsetof() and sizeof() will give different results in the
> presence of alignment padding.
> 
> https://godbolt.org/z/rqnxTK
> 
> I think, grepping at random, that at least struct scsi_vpd is like this,
> size is 24 but data[] starts at offset 20.
> 
> 	struct scsi_vpd {
> 		struct rcu_head	rcu;
> 		int		len;
> 		unsigned char	data[];
> 	};

For another standards 'brain-fart' consider:
	x = malloc(offsetof(struct scsi_vpd, data[count]));

Since offsetof() is defined to return a compile-time constant
(hi Microsoft) this is illegal unless 'count' is also a
compile-time constant.
(It ought to be defined to be constant if the field is constant.)

If count < 4 then *x = *y will also write past the end of x.
Such structure assignments should be compile-time errors.

	David

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