Debugging for Trace Flags

Good thing I have a test machine, because I just crashed SQL Server again. Prepare yourselves for (yet another) post about doing horrible things to the database for fun and profit more fun.

Obligatory “blah blah serious warning blah blah DON’T DO THIS IN PRODUCTION blah blah or you’ll go blind blah blah.” Ok, let’s get started! I have a documented obsession with undocumented trace flags and have long wanted to use the debugger to discover new ones. I just didn’t comprehend what I saw in windbg until I learned about registers and how a program actually runs. Thanks, Crash Course Computing!

Armed with an incomplete understanding of fundamentals and a googled list of windbg commands, I hypothesized that if I turned on some trace flags for a query, they would eventually be passed through one of the registers. From there, I could work out the mechanisms involved and maybe even how to find new flags!

Running a query with a ridiculously long compile time let me capture optimization call stacks with PerfView. The common root of those seemed to be sqllang!CSQLSource::Transform, so I set a breakpoint on it. F11 let me step into the various classes [I don’t even know if I’m using the right terms here] called, and I eventually hit a lucky break.

Behold, sqldk!CSessionTraceFlags::CheckSessionTraceInternal

Clues just don’t get more obvious than that.

Setting a breakpoint on that function [module? class?] revealed that it gets called a lot. I kept F11-ing through the function while watching the registers. I had turned on several trace flags and was looking for their hex forms, but didn’t see them. Eventually, I ran across a call to CheckSessionTraceInternal originating from sqllang!CExecLvlIntCtxt::FDontExpandPersistedCC. “Aha!” I thought (and might have said out loud) “I know what trace flag prevents expanding persisted computed columns.” A glance at my favorite list of trace flags confirmed what I remembered; I was looking at TF 176. The windows calculator has a handy hex converter in its “Progammer” setting, and I quickly determined I would be looking for “b0” in a register.


The pattern soon became obvious. CheckSessionTraceInternal is called whenever a function checks whether a trace flag is turned on. Luckily, when CheckSessionTraceInternal is called, the value of the trace flag it’s checking is passed through register rdx (I think technically edx, but don’t ask me the difference yet) and then stored in r10.

I spent a dozen-plus hours hitting breakpoints on CheckSessionTraceInternal, and made a long list of interesting flags along with the function [method?] checking the flag. I even saw my new friend 8672.

Well, that was the long rambling version of how I discovered this. TLDR is that a breakpoint on sqldk!CSessionTraceFlags ::CheckSessionTraceInternal exposes the trace flag being checked in a register. And of course, Paul White found it first.