On Thu, Jul 24, 2014 at 12:25 PM, Alexei Starovoitov <ast@xxxxxxxxxxxx> wrote: > On Thu, Jul 24, 2014 at 11:25 AM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote: >> On Thu, Jul 17, 2014 at 9:20 PM, Alexei Starovoitov <ast@xxxxxxxxxxxx> wrote: >>> Safety of eBPF programs is statically determined by the verifier, which detects: >>> - loops >>> - out of range jumps >>> - unreachable instructions >>> - invalid instructions >>> - uninitialized register access >>> - uninitialized stack access >>> - misaligned stack access >>> - out of range stack access >>> - invalid calling convention >> >> Is there something that documents exactly what conditions an eBPF >> program must satisfy in order to be considered valid? > > I did a writeup in the past on things that verifiers checks and gave it > to internal folks to review. Guys have said that now they understand very > well how it works, but in reality it didn't help at all to write valid programs. > What worked is 'verification trace' = the instruction by instruction dump > of verifier state while it's analyzing the program. > I gave few simple examples of it in > 'Understanding eBPF verifier messages' section: > https://git.kernel.org/cgit/linux/kernel/git/ast/bpf.git/diff/Documentation/networking/filter.txt?id=b22459133b9f52d2176c8c0f8b5eb036478a40c9 > Every example there is what "program must satisfy to be valid"... > > Therefore I'm addressing two things: > 1. how verifier works and what it checks for. > that is described in 'eBPF verifier' section of the doc and > in 200 lines of comments inside verifier.c That doc is pretty good. I'll try to read it carefully soon. Sorry for the huge delay here -- I've been on vacation. --Andy > 2. how to write valid programs > that's more important one, since it's a key to happy users. > 'verification trace' is the first step. I'm planning to add debug info and > user space tool that points out to line in C instead of assembler trace. > In other words to bring errors to user as early as possible during > compilation process. > This is not a concern when programs are written in assembler, > since the programs will be much shorter and thought through by > the author. However I don't think there will be too many users > willing to understand ebpf assembler. > > I suspect you're more concerned about #1 at this point whereas > I'm concerned about #2. -- Andy Lutomirski AMA Capital Management, LLC -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
