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

From: David Laight
Date: Sat Sep 19 2020 - 11:15:14 EST

From: Arvind Sankar
> Sent: 18 September 2020 23:40
> Ouch, offsetof() and sizeof() will give different results in the
> presence of alignment padding.
> 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.


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