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Commit 031987b6 authored by Theodore Ts'o's avatar Theodore Ts'o Committed by Greg Kroah-Hartman
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random: use lockless method of accessing and updating f->reg_idx 



commit 92e75428ffc90e2a0321062379f883f3671cfebe upstream.

Linus pointed out that there is a much more efficient way of avoiding
the problem that we were trying to address in commit 9dfa7bba35ac0:
"fix race in drivers/char/random.c:get_reg()".

Signed-off-by: default avatarTheodore Ts'o <tytso@mit.edu>
Cc: Michael Schmitz <schmitzmic@gmail.com>
Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
parent db203ab2

drivers/char/random.c

+6 −6
Original line number Diff line number Diff line
@@ -886,15 +886,15 @@ static void add_interrupt_bench(cycles_t start) 
static __u32 get_reg(struct fast_pool *f, struct pt_regs *regs)

{

	__u32 *ptr = (__u32 *) regs;

	unsigned long flags;

	unsigned int idx;



	if (regs == NULL)

		return 0;

	local_irq_save(flags);

	if (f->reg_idx >= sizeof(struct pt_regs) / sizeof(__u32))

		f->reg_idx = 0;

	ptr += f->reg_idx++;

	local_irq_restore(flags);

	idx = READ_ONCE(f->reg_idx);

	if (idx >= sizeof(struct pt_regs) / sizeof(__u32))

		idx = 0;

	ptr += idx++;

	WRITE_ONCE(f->reg_idx, idx);

	return *ptr;

}