[PATCH ethtool v2] ethtool: fix EEPROM byte write

["Tomasz Moń" <tomasz.mon@camlingroup.com>]

ethtool since version 1.8 supports EEPROM byte write:
  # ethtool -E DEVNAME [ magic N ] [ offset N ] [ value N ]

ethtool 2.6.33 added EEPROM block write:
  # ethtool -E ethX [ magic N ] [ offset N ] [ length N ] [ value N ]

EEPROM block write introduced in 2.6.33 is backwards compatible, i.e.
when value is specified the length is forced to 1 (commandline length
value is ignored).

The byte write behaviour changed in ethtool 5.9 where the value write
only works when value parameter is specified together with length 1.
While byte writes to any offset other than 0, without length 1, simply
fail with "offset & length out of bounds" error message, writing value
to offset 0 basically erased whole EEPROM. That is, the provided byte
value was written at offset 0, but the rest of the EEPROM was set to 0.

Fix the issue by setting length to 1 when value is specified and length
is omitted. Exit with error if length is specified to value other than 1
and value is specified.

Fixes: 923c3f51c444 ("ioctl: check presence of eeprom length argument properly")
Signed-off-by: Tomasz Moń <tomasz.mon@camlingroup.com>
---
changes in v2:
  - set the length to 1 only if not specified by user
  - exit with error if length is not 1

v1: https://lore.kernel.org/netdev/20220819062933.1155112-1-tomasz.mon@camlingroup.com/
---
 ethtool.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/ethtool.c b/ethtool.c
index 89613ca..7b400da 100644
--- a/ethtool.c
+++ b/ethtool.c
@@ -3529,12 +3529,16 @@ static int do_seeprom(struct cmd_context *ctx)
 		return 74;
 	}
 
-	if (seeprom_value_seen)
+	if (seeprom_value_seen && !seeprom_length_seen)
 		seeprom_length = 1;
-
-	if (!seeprom_length_seen)
+	else if (!seeprom_length_seen)
 		seeprom_length = drvinfo.eedump_len;
 
+	if (seeprom_value_seen && (seeprom_length != 1)) {
+		fprintf(stderr, "value requires length 1\n");
+		return 1;
+	}
+
 	if (drvinfo.eedump_len < seeprom_offset + seeprom_length) {
 		fprintf(stderr, "offset & length out of bounds\n");
 		return 1;
-- 
2.25.1