linux-kernel - Re: [PATCH] sd: close hole in > 2T device rejection when !CONFIG

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Date:   Mon, 27 Feb 2017 18:57:44 +0000
From:   Bart Van Assche <Bart.VanAssche@...disk.com>
To:     "jejb@...ux.vnet.ibm.com" <jejb@...ux.vnet.ibm.com>,
        "steve.magnani@...idescorp.com" <steve.magnani@...idescorp.com>,
        "martin.petersen@...cle.com" <martin.petersen@...cle.com>
CC:     "linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "steve@...idescorp.com" <steve@...idescorp.com>
Subject: Re: [PATCH] sd: close hole in > 2T device rejection when
 !CONFIG_LBDAF

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