On 2019-12-03, Petr Mladek <pmladek@xxxxxxxx> wrote: >> Add the reader implementation for the new ringbuffer. >> >> Signed-off-by: John Ogness <john.ogness@xxxxxxxxxxxxx> >> --- >> kernel/printk/printk_ringbuffer.c | 234 ++++++++++++++++++++++++++++++ >> kernel/printk/printk_ringbuffer.h | 12 +- >> 2 files changed, 245 insertions(+), 1 deletion(-) >> >> diff --git a/kernel/printk/printk_ringbuffer.c b/kernel/printk/printk_ringbuffer.c >> index 09c32e52fd40..f85762713583 100644 >> --- a/kernel/printk/printk_ringbuffer.c >> +++ b/kernel/printk/printk_ringbuffer.c >> @@ -674,3 +674,237 @@ void prb_commit(struct prb_reserved_entry *e) >> local_irq_restore(e->irqflags); >> } >> EXPORT_SYMBOL(prb_commit); >> + >> +/* >> + * Given @blk_lpos, return a pointer to the raw data from the data block >> + * and calculate the size of the data part. A NULL pointer is returned >> + * if @blk_lpos specifies values that could never be legal. >> + * >> + * This function (used by readers) performs strict validation on the lpos >> + * values to possibly detect bugs in the writer code. A WARN_ON_ONCE() is >> + * triggered if an internal error is detected. >> + */ >> +static char *get_data(struct prb_data_ring *data_ring, >> + struct prb_data_blk_lpos *blk_lpos, >> + unsigned long *data_size) >> +{ >> + struct prb_data_block *db; >> + >> + if (blk_lpos->begin == INVALID_LPOS && >> + blk_lpos->next == INVALID_LPOS) { >> + /* descriptor without a data block */ >> + return NULL; >> + } else if (DATA_WRAPS(data_ring, blk_lpos->begin) == >> + DATA_WRAPS(data_ring, blk_lpos->next)) { >> + /* regular data block */ >> + if (WARN_ON_ONCE(blk_lpos->next <= blk_lpos->begin)) >> + return NULL; >> + db = to_block(data_ring, blk_lpos->begin); >> + *data_size = blk_lpos->next - blk_lpos->begin; >> + >> + } else if ((DATA_WRAPS(data_ring, blk_lpos->begin) + 1 == >> + DATA_WRAPS(data_ring, blk_lpos->next)) || >> + ((DATA_WRAPS(data_ring, blk_lpos->begin) == >> + DATA_WRAPS(data_ring, -1UL)) && >> + (DATA_WRAPS(data_ring, blk_lpos->next) == 0))) { > > I am a bit confused. I would expect that (-1UL + 1) = 0. So the second > condition after || looks just like a special variant of the first > valid condition. > > Or do I miss anything? Is there a problems with type casting? Sorry, this code deserves a comment. Here we are only comparing the number of wraps. For a wrapping data block, @begin will be 1 wrap less than @next. The first part of the check is checking the typical case, making sure that: 1 + WRAPS(@begin) == WRAPS(@next) There is also the case when the lpos overflows. In that case the number of wraps starts over at zero (without having overflowed). (Note: The lpos overflows, _not_ the number of wraps. This is why the first check is not enough.) In this case, the number of wraps of the highest possible lpos value (-1UL) should be the same as the number of wraps of @begin. And the number of wraps of @next should be 0. The simplified pseudo-code check is: WRAPS(@begin) == WRAPS(-1UL) && WRAPS(@next) == 0 >> + /* wrapping data block */ >> + db = to_block(data_ring, 0); >> + *data_size = DATA_INDEX(data_ring, blk_lpos->next); >> + >> + } else { >> + WARN_ON_ONCE(1); >> + return NULL; >> + } >> + >> + /* A valid data block will always be aligned to the ID size. */ >> + if (WARN_ON_ONCE(blk_lpos->begin != >> + ALIGN(blk_lpos->begin, sizeof(db->id))) || >> + WARN_ON_ONCE(blk_lpos->next != >> + ALIGN(blk_lpos->next, sizeof(db->id)))) { >> + return NULL; >> + } >> + >> + /* A valid data block will always have at least an ID. */ >> + if (WARN_ON_ONCE(*data_size < sizeof(db->id))) >> + return NULL; >> + >> + /* Subtract descriptor ID space from size. */ >> + *data_size -= sizeof(db->id); >> + >> + return &db->data[0]; >> +} >> + >> +/* Given @blk_lpos, copy an expected @len of data into the provided buffer. */ >> +static bool copy_data(struct prb_data_ring *data_ring, >> + struct prb_data_blk_lpos *blk_lpos, u16 len, char *buf, >> + unsigned int buf_size) >> +{ >> + unsigned long data_size; >> + char *data; >> + >> + /* Caller might not want the data. */ >> + if (!buf || !buf_size) >> + return true; >> + >> + data = get_data(data_ring, blk_lpos, &data_size); >> + if (!data) >> + return false; >> + >> + /* Actual cannot be less than expected. */ >> + if (WARN_ON_ONCE(data_size < len)) >> + return false; > > I do not have a good feeling that the record gets lost here. > > I could imagine that a writer would reserve more space than > needed in the end. Then it would want to modify desc.info.text_len > and could do a mistake. > > By other words, I would expect a bug on the writer side here. > And I would try to preserve the data by calling: > > pr_warn_once("Wrong data_size (%lu) for data: %.*s\n", data_size, > data_size, data); > > Well, I do not resist on it. WARN_ON_ONCE() is fine as well. Since readers will run in their own kthread, the WARN_ON_ONCE() will not be sufficient to identify the bug. Attempting to print the bad string would help. (Although I expect we will not hit these WARN_ON's since we are the ones implementing printk.) John Ogness _______________________________________________ kexec mailing list kexec@xxxxxxxxxxxxxxxxxxx http://lists.infradead.org/mailman/listinfo/kexec