bootm: Fix Linux silent console on newer kernels

Linux determines its console based on several sources:

1. the console command line parameter
2. device tree (e.g. /chosen/stdout-path)
3. various other board- and arch-specific sources

If the console parameter specifies a real console (e.g. ttyS0) then that is
used as /dev/console. However, if it does not specify a real console (e.g.
ttyDoesntExist) then *nothing* will be used as /dev/console.
Reading/writing it will return ENODEV. Additionally, no other source will
be used as a console source.

Linux commit ab4af56ae250 ("printk/console: Allow to disable console output
by using console="" or console=null") recently changed the semantics of the
parameter. Previously, specifying console="" would be treated like
specifying some other bad console. This commit changed things so that it
added /dev/ttynull as a console (if available).  However, it also allows
for other console sources. If the device tree specifies a console (such as
if U-Boot and Linux share a device tree), then it will be used in addition
to /dev/ttynull. This can result in a non-silent console.

To avoid this, explicitly set ttynull as the console. This will disable
other console sources. If CONFIG_NULL_TTY is disabled, then this will have
the same behavior as in the past (no output, and writing /dev/console
returns ENODEV).

[1] and [2] have additional background on this kernel change.

[1] https://lore.kernel.org/all/20201006025935.GA597@jagdpanzerIV.localdomain/
[2] https://lore.kernel.org/all/20201111135450.11214-1-pmladek@suse.com/

Signed-off-by: Sean Anderson <sean.anderson@seco.com>

1 files changed, 9 insertions, 7 deletions

diff --git a/boot/bootm.c b/boot/bootm.c
index 714406ab66..dfa65f125e 100644
--- a/boot/bootm.c
+++ b/boot/bootm.c
@@ -498,7 +498,8 @@ ulong bootm_disable_interrupts(void)
 }
 
 #define CONSOLE_ARG		"console="
-#define CONSOLE_ARG_SIZE	sizeof(CONSOLE_ARG)
+#define NULL_CONSOLE		(CONSOLE_ARG "ttynull")
+#define CONSOLE_ARG_SIZE	sizeof(NULL_CONSOLE)
 
 /**
  * fixup_silent_linux() - Handle silencing the linux boot if required
@@ -550,21 +551,22 @@ static int fixup_silent_linux(char *buf, int maxlen)
 			char *end = strchr(start, ' ');
 			int start_bytes;
 
-			start_bytes = start - cmdline + CONSOLE_ARG_SIZE - 1;
+			start_bytes = start - cmdline;
 			strncpy(buf, cmdline, start_bytes);
+			strncpy(buf + start_bytes, NULL_CONSOLE, CONSOLE_ARG_SIZE);
 			if (end)
-				strcpy(buf + start_bytes, end);
+				strcpy(buf + start_bytes + CONSOLE_ARG_SIZE - 1, end);
 			else
-				buf[start_bytes] = '\0';
+				buf[start_bytes + CONSOLE_ARG_SIZE] = '\0';
 		} else {
-			sprintf(buf, "%s %s", cmdline, CONSOLE_ARG);
+			sprintf(buf, "%s %s", cmdline, NULL_CONSOLE);
 		}
 		if (buf + strlen(buf) >= cmdline)
 			return -ENOSPC;
 	} else {
-		if (maxlen < sizeof(CONSOLE_ARG))
+		if (maxlen < CONSOLE_ARG_SIZE)
 			return -ENOSPC;
-		strcpy(buf, CONSOLE_ARG);
+		strcpy(buf, NULL_CONSOLE);
 	}
 	debug("after silent fix-up: %s\n", buf);