lib: packing: demote truncation error in pack() to a warning in __pack()

JIRA: https://issues.redhat.com/browse/RHEL-92666

Upstream commit(s):
commit 48c2752
Author: Vladimir Oltean <vladimir.oltean@nxp.com>
Date:   Tue Dec 10 12:27:11 2024 -0800

    lib: packing: demote truncation error in pack() to a warning in __pack()

    Most of the sanity checks in pack() and unpack() can be covered at
    compile time. There is only one exception, and that is truncation of the
    uval during a pack() operation.

    We'd like the error-less __pack() to catch that condition as well. But
    at the same time, it is currently the responsibility of consumer drivers
    (currently just sja1105) to print anything at all when this error
    occurs, and then discard the return code.

    We can just print a loud warning in the library code and continue with
    the truncated __pack() operation. In practice, having the warning is
    very important, see commit 24deec6 ("net: dsa: sja1105: disallow
    C45 transactions on the BASE-TX MDIO bus") where the bug was caught
    exactly by noticing this print.

    Add the first print to the packing library, and at the same time remove
    the print for the same condition from the sja1105 driver, to avoid
    double printing.

    Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
    Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
    Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
    Link: https://patch.msgid.link/20241210-packing-pack-fields-and-ice-implementation-v10-2-ee56a47479ac@intel.com
    Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Signed-off-by: Petr Oros <poros@redhat.com>

Author: orosp

drivers/net/dsa/sja1105/sja1105_static_config.c

@@ -26,12 +26,8 @@ void sja1105_pack(void *buf, const u64 *val, int start, int end, size_t len)
 		pr_err("Start bit (%d) expected to be larger than end (%d)\n",
 		       start, end);
 	} else if (rc == -ERANGE) {
-		if ((start - end + 1) > 64)
-			pr_err("Field %d-%d too large for 64 bits!\n",
-			       start, end);
-		else
-			pr_err("Cannot store %llx inside bits %d-%d (would truncate)\n",
-			       *val, start, end);
+		pr_err("Field %d-%d too large for 64 bits!\n",
+		       start, end);
 	}
 	dump_stack();
 }

lib/packing.c

@@ -59,8 +59,17 @@ static void __pack(void *pbuf, u64 uval, size_t startbit, size_t endbit,
 	 */
 	int plogical_first_u8 = startbit / BITS_PER_BYTE;
 	int plogical_last_u8 = endbit / BITS_PER_BYTE;
+	int value_width = startbit - endbit + 1;
 	int box;
 
+	/* Check if "uval" fits in "value_width" bits.
+	 * The test only works for value_width < 64, but in the latter case,
+	 * any 64-bit uval will surely fit.
+	 */
+	WARN(value_width < 64 && uval >= (1ull << value_width),
+	     "Cannot store 0x%llx inside bits %zu-%zu - will truncate\n",
+	     uval, startbit, endbit);
+
 	/* Iterate through an idealistic view of the pbuf as an u64 with
 	 * no quirks, u8 by u8 (aligned at u8 boundaries), from high to low
 	 * logical bit significance. "box" denotes the current logical u8.
@@ -143,29 +152,14 @@ static void __pack(void *pbuf, u64 uval, size_t startbit, size_t endbit,
 int pack(void *pbuf, u64 uval, size_t startbit, size_t endbit, size_t pbuflen,
 	 u8 quirks)
 {
-	/* width of the field to access in the pbuf */
-	u64 value_width;
-
 	/* startbit is expected to be larger than endbit, and both are
 	 * expected to be within the logically addressable range of the buffer.
 	 */
 	if (unlikely(startbit < endbit || startbit >= BITS_PER_BYTE * pbuflen))
 		/* Invalid function call */
 		return -EINVAL;
 
-	value_width = startbit - endbit + 1;
-	if (unlikely(value_width > 64))
-		return -ERANGE;
-
-	/* Check if "uval" fits in "value_width" bits.
-	 * If value_width is 64, the check will fail, but any
-	 * 64-bit uval will surely fit.
-	 */
-	if (value_width < 64 && uval >= (1ull << value_width))
-		/* Cannot store "uval" inside "value_width" bits.
-		 * Truncating "uval" is most certainly not desirable,
-		 * so simply erroring out is appropriate.
-		 */
+	if (unlikely(startbit - endbit >= 64))
 		return -ERANGE;
 
 	__pack(pbuf, uval, startbit, endbit, pbuflen, quirks);