Good post. Slightly off-topic, but wanted to bring in discussion of user-mode debugging with mdb.
I for one, would like to see basic source-level debugging added to mdb (option 2 below) for use with user-mode binaries. A side-effect, I would say, of making mdb both a good kernel AND user-mode debugger ;) I see the feature on the wishlist - http://www.opensolaris.org/os/community/mdb/future/ The reality is that mdb ships with Solaris, and thus is usually the only debugger installed at a customer site. Yes, gdb does now ship as Solaris add-on, or can be downloaded, but is an extra step that has to pass through customer IT process (slow!). Moreover, gdb doesn't always work well with SunStudio debug info. And generally, dbx is out of the question, as it comes bundled with SunStudio (yes, now free, but still a large install for just the debugger). Basically, I whole-heartedly put my vote in for mdb supporting the debug formats output by SunStudio compiler chain, and producing a good backtrace with source line numbers. Mdb wouldn't have to display source, step through source; in other words, not a full-featured user-mode debugger. Mostly for user-mode post-mortem analysis. P.S. I do realize the Solaris10 generates core files with program text, some symbol info, for much better "offline" core file analysis. However, I believe the symbol info isn't the same as what SunStudio generates, so perhaps the stack trace won't be complete. Please correct me if I am wrong on this. Regardless, still would be a nice mdb feature. Thanks, Ivan -----Original Message----- From: mdb-discuss-boun...@opensolaris.org [mailto:mdb-discuss-bounces at opensolaris.org] On Behalf Of Michael Shapiro Sent: Friday, February 10, 2006 9:21 PM To: Vin Cc: mdb-discuss at opensolaris.org Subject: Re: [mdb-discuss] source-level kernel debugging? > Does kmdb support source-level kernel debugging? > > Got out of Solaris work for quite sometime now, and just catching up, > with the latest developments. I know earlier debuggers like kadb had > only assembly-level kernel debugging. Not sure what specifically you're looking for, but in general people mean one or more of three different things when they say "source-level debugging": 1. The ability to view data structures in a source-level view corresponding to the data structure definitions of the source language. Both mdb and kmdb offer this feature now. The ::print command can be used to print types, e.g. 0x12345678::print vnode_t and we have other commands like ::list that let you take advantage of source-level structure names, etc. This is all for C only obviously. 2. The ability to correlate program counter offsets to source-level file names and line numbers. We don't offer this feature in either mdb or kmdb today. Typically this feature isn't very useful in the kind of low-level systems debugging we do in the kernel with kmdb and mdb because you're dealing with optimized, production code and your compiler is free to reorder things quite aggressively, making the assembly and source not closely correspond (it is for this reason that most compilers disable some or all optimization when you compile -g to get the offset->line debug information). That said, if someone wants to add DWARF processing for this in a way which is reasonable I wouldn't be opposed to it. 3. The ability to perform simple breakpoints and breakpoint/step based on source lines, implemented on top of #2. This is pretty much entirely out of whack with multi-threaded programming in general in my view, and certainly with the low-level MT systems debugging that mdb/kmdb are designed for. A much better way to approach this is with DTrace, where you can dynamically examine MT systems behavior in a way that is infinitely more flexible and sophisticated than breakpoint-based debugging. In general, we use mdb for postmortem analysis and DTrace for live system analysis. Both of these tools and their accompanying techniques, described in the respective docs.sun.com manuals, are worth investing the time to learn. -Mike -- Mike Shapiro, Solaris Kernel Development. blogs.sun.com/mws/ _______________________________________________ mdb-discuss mailing list mdb-discuss at opensolaris.org
