forgot to mention... Of course, I am not sure that UPROBE_KERN_CTR can actually work, there are a lot of details. But if it can, then we can also make ->ref_ctr_offset a consumer property. On 07/03, Oleg Nesterov wrote: > > On 07/03, Ravi Bangoria wrote: > > > > > OK, and how exactly they update the counter? I mean, can we assume that, say, > > > bcc or systemtap can only increment or decrement it? > > > > I don't think we can assume anything here because this is all in user's > > control. User can even manually go and update the counter by directly > > hooking into the memory. > > Then how this all can work? I understand that user-space can do anything with > this counter, but we do not care if it does something wrong, say nullifies the > ctr incremented by kernel. > > I don't understand this. I think that if a user registers uprobe with > ->ref_ctr_offset != 0 we can safely assume that this is a counter, and we do > not care if userspace corrupts it. > > > > If yes, perhaps we can simplify the kernel code... > > > > Sure, let me know if you have any better idea. > > Can't we (ab)use the most significant bit in this counter? > > To simplify, lets suppose for the moment that 2 different uprobes can't have > the same ->ref_ctr_offset. Then we can do something like > > #define UPROBE_KERN_CTR (SHRT_MAX + 1) // MSB > > install_breakpoint: > > for (each valid_ref_ctr_vma which maps uprobe->ref_ctr_offset) > *ctr_ptr |= UPROBE_KERN_CTR; > > set_swbp(); > > and > > remove_breakpoint: > > for (each valid_ref_ctr_vma which maps uprobe->ref_ctr_offset) > *ctr_ptr &= ~UPROBE_KERN_CTR; > > set_orig_insn(); > > IOW, we increment/decrement by UPROBE_KERN_CTR, not by 1. But this way the > "increment" is idempotent, we do not care if "|=" or "&=" was applied more than > once, we do not need to record the fact that the counter was already incremented, > and inc/dec are always balanced. > > > Now, lets recall that multiple uprobes can share the same counter. install_breakpoint() > is still fine, and we only need to add the additional code into remove_breakpoint: > > for (each uprobe with the same inode and ref_ctr_offset) > if (filter_chain(uprobe)) > goto keep_ctr; > > for (each valid_ref_ctr_vma which maps uprobe->ref_ctr_offset) > *ctr_ptr &= ~UPROBE_KERN_CTR; > > keep_ctr: > set_orig_insn(); > > > Just an idea. > > What do you think? > > Oleg.
