On 7/30/2020 1:49 PM, Matthew Wilcox wrote: > On Thu, Jul 30, 2020 at 01:35:51PM -0400, Steven Sistare wrote: >> mshare + VA reservation is another possible solution. >> >> Or MADV_DOEXEC alone, which is ready now. I hope we can get back to reviewing that. > > We are. This is the part of the review process where we explore other > solutions to the problem. > >>>> Also, we need to support updating legacy processes that already created anon segments. >>>> We inject code that calls MADV_DOEXEC for such segments. >>> >>> Yes, I was assuming you'd inject code that called mshare(). >> >> OK, mshare works on existing memory and builds a new vma. > > Actually, reparents an existing VMA, and reuses the existing page tables. > >>> Actually, since you're injecting code, why do you need the kernel to >>> be involved? You can mmap the new executable and any libraries it depends >>> upon, set up a new stack and jump to the main() entry point, all without >>> calling exec(). I appreciate it'd be a fair amount of code, but it'd all >>> be in userspace and you can probably steal / reuse code from ld.so (I'm >>> not familiar with the details of how setting up an executable is done). >> >> Duplicating all the work that the kernel and loader do to exec a process would >> be error prone, require ongoing maintenance, and be redundant. Better to define >> a small kernel extension and leave exec to the kernel. > > Either this is a one-off kind of thing, in which case it doesn't need > ongoing maintenance, or it's something with broad applicability, in > which case it can live as its own userspace project. It could even > start off life as part of qemu and then fork into its own project. exec will be enhanced over time in the kernel. A separate user space implementation would need to track that. Reimplementing exec in userland would be a big gross mess. Not a good solution when we have simple and concise ways of solving the problem. > The idea of tagging an ELF executable to say "I can cope with having > chunks of my address space provided to me by my executor" is ... odd. I don't disagree. But it is useful. We already pass a block of data containing environment variables and arguments from one process to the next. Preserving additional segments is not a big leap from there. - Steve
