[PATCH] powerpc/xmon: Lower limits on nidump and ndump

[PATCH] powerpc/xmon: Lower limits on nidump and ndump

[Michael Ellerman]

In xmon we have two variables that are used by the dump commands.
There's ndump which is the number of bytes to dump using 'd', and
nidump which is the number of instructions to dump using 'di'.

ndump starts as 64 and nidump starts as 16, but both can be set by the
user.

It's fairly common to be pasting addresses into xmon when trying to
debug something, and if you inadvertently double paste an address like
so:

  0:mon> di c000000002101f6c c000000002101f6c

The second value is interpreted as the number of instructions to dump.

Luckily it doesn't dump 13 quintrillion instructions, the value is
limited to MAX_DUMP (128K). But as each instruction is dumped on a
single line, that's still a lot of output. If you're on a slow console
that can take multiple minutes to print. If you were "just popping in
and out of xmon quickly before the RCU/hardlockup detector fires" you
are now having a bad day.

Things are not as bad with 'd' because we print 16 bytes per line, so
it's fewer lines. But it's still quite a lot.

So shrink the maximum for 'd' to 64K (one page), which is 4096 lines.
For 'di' add a new limit which is the above / 4 - because instructions
are 4 bytes, meaning again we can dump one page.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
---
 arch/powerpc/xmon/xmon.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/arch/powerpc/xmon/xmon.c b/arch/powerpc/xmon/xmon.c
index e8c84d265602..722bf7ed0eda 100644
--- a/arch/powerpc/xmon/xmon.c
+++ b/arch/powerpc/xmon/xmon.c
@@ -81,8 +81,9 @@ static bool xmon_is_ro = IS_ENABLED(CONFIG_XMON_DEFAULT_RO_MODE);
 
 static unsigned long adrs;
 static int size = 1;
-#define MAX_DUMP (128 * 1024)
+#define MAX_DUMP (64 * 1024)
 static unsigned long ndump = 64;
+#define MAX_IDUMP (MAX_DUMP >> 2)
 static unsigned long nidump = 16;
 static unsigned long ncsum = 4096;
 static int termch;
@@ -2756,8 +2757,8 @@ dump(void)
 		scanhex(&nidump);
 		if (nidump == 0)
 			nidump = 16;
-		else if (nidump > MAX_DUMP)
-			nidump = MAX_DUMP;
+		else if (nidump > MAX_IDUMP)
+			nidump = MAX_IDUMP;
 		adrs += ppc_inst_dump(adrs, nidump, 1);
 		last_cmd = "di\n";
 	} else if (c == 'l') {
-- 
2.21.1

Re: [PATCH] powerpc/xmon: Lower limits on nidump and ndump

[Michael Ellerman]

On Wed, 2020-02-19 at 11:00:07 UTC, Michael Ellerman wrote:
> In xmon we have two variables that are used by the dump commands.
> There's ndump which is the number of bytes to dump using 'd', and
> nidump which is the number of instructions to dump using 'di'.
> 
> ndump starts as 64 and nidump starts as 16, but both can be set by the
> user.
> 
> It's fairly common to be pasting addresses into xmon when trying to
> debug something, and if you inadvertently double paste an address like
> so:
> 
>   0:mon> di c000000002101f6c c000000002101f6c
> 
> The second value is interpreted as the number of instructions to dump.
> 
> Luckily it doesn't dump 13 quintrillion instructions, the value is
> limited to MAX_DUMP (128K). But as each instruction is dumped on a
> single line, that's still a lot of output. If you're on a slow console
> that can take multiple minutes to print. If you were "just popping in
> and out of xmon quickly before the RCU/hardlockup detector fires" you
> are now having a bad day.
> 
> Things are not as bad with 'd' because we print 16 bytes per line, so
> it's fewer lines. But it's still quite a lot.
> 
> So shrink the maximum for 'd' to 64K (one page), which is 4096 lines.
> For 'di' add a new limit which is the above / 4 - because instructions
> are 4 bytes, meaning again we can dump one page.
> 
> Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

Applied to powerpc next.

https://git.kernel.org/powerpc/c/d64c7dbb4d98306b794401ca924ad053f84b59f8

cheers