Date: Fri, 2 Feb 2007 01:25:58 GMT From: Ravi Murty<ravi.murty@intel.com> To: freebsd-gnats-submit@FreeBSD.org Subject: amd64/108673: kgdb doesn't work very well very long Message-ID: <200702020125.l121Pw1I079198@www.freebsd.org> Resent-Message-ID: <200702020130.l121UESt045864@freefall.freebsd.org>
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>Number: 108673 >Category: amd64 >Synopsis: kgdb doesn't work very well very long >Confidential: no >Severity: serious >Priority: low >Responsible: freebsd-amd64 >State: open >Quarter: >Keywords: >Date-Required: >Class: sw-bug >Submitter-Id: current-users >Arrival-Date: Fri Feb 02 01:30:14 GMT 2007 >Closed-Date: >Last-Modified: >Originator: Ravi Murty >Release: 6.1-release >Organization: Intel >Environment: FreeBSD 6.1-RELEASE FreeBSD 6.1-RELEASE #17: Thu Feb 16:57:46 PST 2007 >Description: Here's what I think might be happening. 1. At the loader prompt I enter "boot -d" to break into the debugger because I want to start running the debugger from mi_startup. 2. Kgdb connects on the host and I can step, but it crashes or doesn't work very well. We started to look at things like gdb_txbuf and noticed that a strage 0x78787878 (0x78 is lower case 'x') appears where gdb_txbuf is supposed to be. 3. We looked at the source code a bit and realized that gdb_txbuf is sized to be 500 bytes (defined by GDB_BUFSZ in amd64/include/gdb_machdep.h). So here's the problem: 1. When kgdb running on the host connects it runs a query to the target to request all the registers. In amd64's case, that's 56 total registers with 49 of size 8 bytes and 7 of size 4. Since we convert from hex nibbles to char's, we expect it to store 16 char's per 8 byte register and 8 char's for 4 byte registers. Simple math tells us that is (16 * 49 + 8 * 7 = 840). So we need atleast 840B in the buffer to return the result of the query. 2. Indeed when we look at what's at gbd_txbuf on the target (via kgdb), we notice that 0x78787878 appears until location 839. It is 0x78787878 because that happens to be "xxxx" for registers that are undefined (see gdb_packet.c) Therefore, shouldn't GDB_BUFSZ be > 840B ? Thanks Ravi >How-To-Repeat: Run kgdb and connect to a machine running 6.1-Release for amd64. Drop to the loader prompt on boot and type "boot -d" and connect to the debugger (gdb backend). Try and step through the code after this point. >Fix: See "Full description". >Release-Note: >Audit-Trail: >Unformatted:
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