Re: [PATCH bpf-next v2 6/8] bpftool: Add LLVM as default library for disassembling JIT-ed programs

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On 9/11/22 10:14 PM, Quentin Monnet wrote:
To disassemble instructions for JIT-ed programs, bpftool has relied on
the libbfd library. This has been problematic in the past: libbfd's
interface is not meant to be stable and has changed several times. For
building bpftool, we have to detect how the libbfd version on the system
behaves, which is why we have to handle features disassembler-four-args
and disassembler-init-styled in the Makefile. When it comes to shipping
bpftool, this has also caused issues with several distribution
maintainers unwilling to support the feature (see for example Debian's
page for binutils-dev, which ships libbfd: "Note that building Debian
packages which depend on the shared libbfd is Not Allowed." [0]).

For these reasons, we add support for LLVM as an alternative to libbfd
for disassembling instructions of JIT-ed programs. Thanks to the
preparation work in the previous commits, it's easy to add the library
by passing the relevant compilation options in the Makefile, and by
adding the functions for setting up the LLVM disassembler in file
jit_disasm.c.

Could you add more context around the LLVM lib? The motivation is that libbfd's
interface is not meant to be stable and has changed several times. How does this
look on the LLVM's library side? Also, for the 2nd part, what is Debian's stance
related to the LLVM lib? Would be good if both is explained in the commit message.
Right now it mainly reads 'that libbfd has all these issues, so we're moving to
something else', so would be good to provide more context to the ready why the
'something else' is better than current one.

Naturally, the display of disassembled instructions comes with a few
minor differences. Here is a sample output with libbfd (already
supported before this patch):

     # bpftool prog dump jited id 56
     bpf_prog_6deef7357e7b4530:
        0:   nopl   0x0(%rax,%rax,1)
        5:   xchg   %ax,%ax
        7:   push   %rbp
        8:   mov    %rsp,%rbp
        b:   push   %rbx
        c:   push   %r13
        e:   push   %r14
       10:   mov    %rdi,%rbx
       13:   movzwq 0xb4(%rbx),%r13
       1b:   xor    %r14d,%r14d
       1e:   or     $0x2,%r14d
       22:   mov    $0x1,%eax
       27:   cmp    $0x2,%r14
       2b:   jne    0x000000000000002f
       2d:   xor    %eax,%eax
       2f:   pop    %r14
       31:   pop    %r13
       33:   pop    %rbx
       34:   leave
       35:   ret

LLVM supports several variants that we could set when initialising the
disassembler, for example with:

     LLVMSetDisasmOptions(*ctx,
                          LLVMDisassembler_Option_AsmPrinterVariant);

but the default printer is used for now. Here is the output with LLVM:

     # bpftool prog dump jited id 56
     bpf_prog_6deef7357e7b4530:
        0:   nopl    (%rax,%rax)
        5:   nop
        7:   pushq   %rbp
        8:   movq    %rsp, %rbp
        b:   pushq   %rbx
        c:   pushq   %r13
        e:   pushq   %r14
       10:   movq    %rdi, %rbx
       13:   movzwq  180(%rbx), %r13
       1b:   xorl    %r14d, %r14d
       1e:   orl     $2, %r14d
       22:   movl    $1, %eax
       27:   cmpq    $2, %r14
       2b:   jne     0x2f
       2d:   xorl    %eax, %eax
       2f:   popq    %r14
       31:   popq    %r13
       33:   popq    %rbx
       34:   leave
       35:   retq

The LLVM disassembler comes as the default choice, with libbfd as a
fall-back.

Of course, we could replace libbfd entirely and avoid supporting two
different libraries. One reason for keeping libbfd is that, right now,
it works well, we have all we need in terms of features detection in the
Makefile, so it provides a fallback for disassembling JIT-ed programs if
libbfd is installed but LLVM is not. The other motivation is that libbfd
supports nfp instruction for Netronome's SmartNICs and can be used to
disassemble offloaded programs, something that LLVM cannot do. If
libbfd's interface breaks again in the future, we might reconsider
keeping support for it.

[0] https://packages.debian.org/buster/binutils-dev

Signed-off-by: Quentin Monnet <quentin@xxxxxxxxxxxxx>
Tested-by: Niklas Söderlund <niklas.soderlund@xxxxxxxxxxxx>

Thanks,
Daniel



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