[ Added Arjan ] On Wed, 2010-01-27 at 02:43 -0800, Linus Torvalds wrote: > > On Wed, 27 Jan 2010, Peter Zijlstra wrote: > > > > Right, so you're going to love uprobes, which does exactly that. The > > current proposal is overwriting the target instruction with an INT3 and > > injecting an extra vma into the target process's address space > > containing the original instruction(s) and possible jumps back to the > > old code stream. > > Just out of interest, how does it handle the threading issue? > > Last I saw, at least some CPU people were _very_ nervous about overwriting > instructions if another CPU might be just about to execute them.
I think the issue was that ring 0 was never meant to do that, where as, ring 3 does it all the time. Doesn't the dynamic library modify its text? -- Steve > > Even the "overwrite only the first byte with 'int3'" made them go "umm, I > need to talk to some core CPU people to see if that's ok". They mumble > about possible CPU errata, I$ coherency, instruction retry etc. > > I realize kprobes does this very thing, but kprobes is esoteric stuff and > doesn't have much choice. In user space, you _could_ do the modification > on a different physical page and then just switch the page table entry > instead, and not get into the whole D$/I$ coherency thing at all. > > Linus
