From: Sebastian Andrzej Siewior Date: Thu, 7 Dec 2023 14:15:15 +0000 Subject: [PATCH 086/134] printk: Adjust mapping for 32bit seq macros Origin: https://www.kernel.org/pub/linux/kernel/projects/rt/6.6/older/patches-6.6.7-rt18.tar.xz Note: This change only applies to 32bit architectures. On 64bit architectures the macros are NOPs. __ulseq_to_u64seq() computes the upper 32 bits of the passed argument value (@ulseq). The upper bits are derived from a base value (@rb_next_seq) in a way that assumes @ulseq represents a 64bit number that is less than or equal to @rb_next_seq. Until now this mapping has been correct for all call sites. However, in a follow-up commit, values of @ulseq will be passed in that are higher than the base value. This requires a change to how the 32bit value is mapped to a 64bit sequence number. Rather than mapping @ulseq such that the base value is the end of a 32bit block, map @ulseq such that the base value is in the middle of a 32bit block. This allows supporting 31 bits before and after the base value, which is deemed acceptable for the console sequence number during runtime. Here is an example to illustrate the previous and new mappings. For a base value (@rb_next_seq) of 2 2000 0000... Before this change the range of possible return values was: 1 2000 0001 to 2 2000 0000 __ulseq_to_u64seq(1fff ffff) => 2 1fff ffff __ulseq_to_u64seq(2000 0000) => 2 2000 0000 __ulseq_to_u64seq(2000 0001) => 1 2000 0001 __ulseq_to_u64seq(9fff ffff) => 1 9fff ffff __ulseq_to_u64seq(a000 0000) => 1 a000 0000 __ulseq_to_u64seq(a000 0001) => 1 a000 0001 After this change the range of possible return values are: 1 a000 0001 to 2 a000 0000 __ulseq_to_u64seq(1fff ffff) => 2 1fff ffff __ulseq_to_u64seq(2000 0000) => 2 2000 0000 __ulseq_to_u64seq(2000 0001) => 2 2000 0001 __ulseq_to_u64seq(9fff ffff) => 2 9fff ffff __ulseq_to_u64seq(a000 0000) => 2 a000 0000 __ulseq_to_u64seq(a000 0001) => 1 a000 0001 [ john.ogness: Rewrite commit message. ] Reported-by: Francesco Dolcini Reported-by: kernel test robot Signed-off-by: Sebastian Andrzej Siewior Signed-off-by: John Ogness Signed-off-by: Sebastian Andrzej Siewior --- kernel/printk/printk_ringbuffer.h | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) --- a/kernel/printk/printk_ringbuffer.h +++ b/kernel/printk/printk_ringbuffer.h @@ -407,7 +407,7 @@ static inline u64 __ulseq_to_u64seq(stru * Also the access to the ring buffer is always safe. */ rb_next_seq = prb_next_seq(rb); - seq = rb_next_seq - ((u32)rb_next_seq - ulseq); + seq = rb_next_seq - (s32)((u32)rb_next_seq - ulseq); return seq; }