• Daniel Borkmann's avatar
    bpf: make jited programs visible in traces · 74451e66
    Daniel Borkmann authored
    Long standing issue with JITed programs is that stack traces from
    function tracing check whether a given address is kernel code
    through {__,}kernel_text_address(), which checks for code in core
    kernel, modules and dynamically allocated ftrace trampolines. But
    what is still missing is BPF JITed programs (interpreted programs
    are not an issue as __bpf_prog_run() will be attributed to them),
    thus when a stack trace is triggered, the code walking the stack
    won't see any of the JITed ones. The same for address correlation
    done from user space via reading /proc/kallsyms. This is read by
    tools like perf, but the latter is also useful for permanent live
    tracing with eBPF itself in combination with stack maps when other
    eBPF types are part of the callchain. See offwaketime example on
    dumping stack from a map.
    This work tries to tackle that issue by making the addresses and
    symbols known to the kernel. The lookup from *kernel_text_address()
    is implemented through a latched RB tree that can be read under
    RCU in fast-path that is also shared for symbol/size/offset lookup
    for a specific given address in kallsyms. The slow-path iteration
    through all symbols in the seq file done via RCU list, which holds
    a tiny fraction of all exported ksyms, usually below 0.1 percent.
    Function symbols are exported as bpf_prog_<tag>, in order to aide
    debugging and attribution. This facility is currently enabled for
    root-only when bpf_jit_kallsyms is set to 1, and disabled if hardening
    is active in any mode. The rationale behind this is that still a lot
    of systems ship with world read permissions on kallsyms thus addresses
    should not get suddenly exposed for them. If that situation gets
    much better in future, we always have the option to change the
    default on this. Likewise, unprivileged programs are not allowed
    to add entries there either, but that is less of a concern as most
    such programs types relevant in this context are for root-only anyway.
    If enabled, call graphs and stack traces will then show a correct
    attribution; one example is illustrated below, where the trace is
    now visible in tooling such as perf script --kallsyms=/proc/kallsyms
    and friends.
      7fff8166889d bpf_clone_redirect+0x80007f0020ed (/lib/modules/4.9.0-rc8+/build/vmlinux)
             f5d80 __sendmsg_nocancel+0xffff006451f1a007 (/usr/lib64/libc-2.18.so)
      7fff816688b7 bpf_clone_redirect+0x80007f002107 (/lib/modules/4.9.0-rc8+/build/vmlinux)
      7fffa0575728 bpf_prog_33c45a467c9e061a+0x8000600020fb (/lib/modules/4.9.0-rc8+/build/vmlinux)
      7fffa07ef1fc cls_bpf_classify+0x8000600020dc (/lib/modules/4.9.0-rc8+/build/vmlinux)
      7fff81678b68 tc_classify+0x80007f002078 (/lib/modules/4.9.0-rc8+/build/vmlinux)
      7fff8164d40b __netif_receive_skb_core+0x80007f0025fb (/lib/modules/4.9.0-rc8+/build/vmlinux)
      7fff8164d718 __netif_receive_skb+0x80007f002018 (/lib/modules/4.9.0-rc8+/build/vmlinux)
      7fff8164e565 process_backlog+0x80007f002095 (/lib/modules/4.9.0-rc8+/build/vmlinux)
      7fff8164dc71 net_rx_action+0x80007f002231 (/lib/modules/4.9.0-rc8+/build/vmlinux)
      7fff81767461 __softirqentry_text_start+0x80007f0020d1 (/lib/modules/4.9.0-rc8+/build/vmlinux)
      7fff817658ac do_softirq_own_stack+0x80007f00201c (/lib/modules/4.9.0-rc8+/build/vmlinux)
      7fff810a2c20 do_softirq+0x80007f002050 (/lib/modules/4.9.0-rc8+/build/vmlinux)
      7fff810a2cb5 __local_bh_enable_ip+0x80007f002085 (/lib/modules/4.9.0-rc8+/build/vmlinux)
      7fff8168d452 ip_finish_output2+0x80007f002152 (/lib/modules/4.9.0-rc8+/build/vmlinux)
      7fff8168ea3d ip_finish_output+0x80007f00217d (/lib/modules/4.9.0-rc8+/build/vmlinux)
      7fff8168f2af ip_output+0x80007f00203f (/lib/modules/4.9.0-rc8+/build/vmlinux)
      7fff81005854 do_syscall_64+0x80007f002054 (/lib/modules/4.9.0-rc8+/build/vmlinux)
      7fff817649eb return_from_SYSCALL_64+0x80007f002000 (/lib/modules/4.9.0-rc8+/build/vmlinux)
             f5d80 __sendmsg_nocancel+0xffff01c484812007 (/usr/lib64/libc-2.18.so)
    Signed-off-by: default avatarDaniel Borkmann <daniel@iogearbox.net>
    Acked-by: default avatarAlexei Starovoitov <ast@kernel.org>
    Cc: linux-kernel@vger.kernel.org
    Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
kallsyms.c 16.5 KB