On 5/20/15 9:05 AM, Andy Lutomirski wrote:
What causes the stack pointer to be right? Is there some reason that
the stack pointer is the same no matter where you are in the generated
code?
that's why I said 'it's _roughly_ expressed in C' this way.
Stack pointer doesn't change. It uses the same stack frame.
I think the more relevant point is that (I think) eBPF never changes
the stack pointer after the prologue (i.e. the stack depth is truly
constant).
ahh, that's what you were referring to.
Yes, there is no alloca(). stack cannot grow and always fixed.
That's critical for safety verification.
On a JIT side though, x64 has ugly div/mod, so JIT is doing
push/pop rax/rdx to compile 'dst_reg /= src_reg' bpf insn.
But that doesn't change 'same stack depth' rule at the time
of bpf_tail_call.
Note, s390 JIT can generate different prologue/epilogue
for every program, so it will likely be doing stack unwind
and jump. Like I was doing in my tail_call_v2 version of x64 jit:
https://git.kernel.org/cgit/linux/kernel/git/ast/bpf.git/diff/arch/x86/net/bpf_jit_comp.c?h=tail_call_v2&id=bfd60c3135c8f010a6497dfc5e7d3070e26ca4d1
In case of interrupt happens sometime during this jumping process
it's also fine. no-red-zone business is very dear to my heart :)
I always keep it in mind when doing assembler/jit changes.
--
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
