Re: [PATCH] sd: close hole in > 2T device rejection when !CONFIG_LBDAF

Bart Van Assche
2017-02-27T19:16:32Z

On Mon, 2017-02-27 at 11:13 -0600, Steve Magnani wrote:
> On 02/27/2017 10:13 AM, Bart Van Assche wrote:
> > Why are the two checks slightly different? Could the same code be used for
> > both checks?
>
> The checks are different because with READ CAPACITY(16) a _really_ huge
> device could report a max LBA so large that left-shifting it causes bits
> to drop off the end. That's not an issue with READ CAPACITY(10) because
> at most the 32-bit LBA reported by the device will become a 35-bit value
> (since the max supported block size is 4096 == (512 << 3)).

Sorry but I still don't understand why the two checks are different. How about
the (untested) patch below? The approach below avoids that the check is
duplicated and - at least in my opinion - results in code that is easier to read.

Thanks,

Bart.

diff --git a/drivers/scsi/sd.c b/drivers/scsi/sd.c
index cb6e68dd6df0..3533d1e46bde 100644
--- a/drivers/scsi/sd.c
+++ b/drivers/scsi/sd.c
@@ -2082,6 +2082,16 @@ static void read_capacity_error(struct scsi_disk *sdkp, struct scsi_device *sdp,
sdkp->capacity = 0; /* unknown mapped to zero - as usual */
}

+/*
+ * Check whether or not logical_to_sectors(sdp, lba) will overflow.
+ */
+static bool lba_too_large(u64 lba, u32 logical_block_size)
+{
+ int shift = sizeof(sector_t) * 8 + 9 - ilog2(logical_block_size);
+
+ return shift >= 0 && shift < 64 && lba >= (1ULL << shift);
+}
+
#define RC16_LEN 32
#if RC16_LEN > SD_BUF_SIZE
#error RC16_LEN must not be more than SD_BUF_SIZE
@@ -2154,7 +2164,7 @@ static int read_capacity_16(struct scsi_disk *sdkp, struct scsi_device *sdp,
return -ENODEV;
}

- if ((sizeof(sdkp->capacity) == 4) && (lba >= 0xffffffffULL)) {
+ if (lba_too_large(lba + 1, sector_size)) {
sd_printk(KERN_ERR, sdkp, "Too big for this kernel. Use a "
"kernel compiled with support for large block "
"devices.\n");
@@ -2243,7 +2253,7 @@ static int read_capacity_10(struct scsi_disk *sdkp, struct scsi_device *sdp,
return sector_size;
}

- if ((sizeof(sdkp->capacity) == 4) && (lba == 0xffffffff)) {
+ if (lba_too_large(lba + 1ULL, sector_size)) {
sd_printk(KERN_ERR, sdkp, "Too big for this kernel. Use a "
"kernel compiled with support for large block "
"devices.\n");
--
2.11.0