bpf: Detect IP == ksym.end as part of BPF program

In the Linux kernel, the following vulnerability has been resolved:

bpf: Detect IP == ksym.end as part of BPF program

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.