• Kumar Kartikeya Dwivedi's avatar
    bpf: Detect IP == ksym.end as part of BPF program · 66d9111f
    Kumar Kartikeya Dwivedi authored
    Now that bpf_throw kfunc is the first such call instruction that has
    noreturn semantics within the verifier, this also kicks in dead code
    elimination in unprecedented ways. For one, any instruction following
    a bpf_throw call will never be marked as seen. Moreover, if a callchain
    ends up throwing, any instructions after the call instruction to the
    eventually throwing subprog in callers will also never be marked as
    seen.
    
    The tempting way to fix this would be to emit extra 'int3' instructions
    which bump the jited_len of a program, and ensure that during runtime
    when a program throws, we can discover its boundaries even if the call
    instruction to bpf_throw (or to subprogs that always throw) is emitted
    as the final instruction in the program.
    
    An example of such a program would be this:
    
    do_something():
    	...
    	r0 = 0
    	exit
    
    foo():
    	r1 = 0
    	call bpf_throw
    	r0 = 0
    	exit
    
    bar(cond):
    	if r1 != 0 goto pc+2
    	call do_something
    	exit
    	call foo
    	r0 = 0  // Never seen by verifier
    	exit	//
    
    main(ctx):
    	r1 = ...
    	call bar
    	r0 = 0
    	exit
    
    Here, if we do end up throwing, the stacktrace would be the following:
    
    bpf_throw
    foo
    bar
    main
    
    In bar, the final instruction emitted will be the call to foo, as such,
    the return address will be the subsequent instruction (which the JIT
    emits as int3 on x86). This will end up lying outside the jited_len of
    the program, thus, when unwinding, we will fail to discover the return
    address as belonging to any program and end up in a panic due to the
    unreliable stack unwinding of BPF programs that we never expect.
    
    To remedy this case, make bpf_prog_ksym_find treat IP == ksym.end as
    part of the BPF program, so that is_bpf_text_address returns true when
    such a case occurs, and we are able to unwind reliably when the final
    instruction ends up being a call instruction.
    Signed-off-by: default avatarKumar Kartikeya Dwivedi <memxor@gmail.com>
    Link: https://lore.kernel.org/r/20230912233214.1518551-12-memxor@gmail.comSigned-off-by: default avatarAlexei Starovoitov <ast@kernel.org>
    66d9111f
core.c 75.4 KB