path: root/changes-v1.17

v1.17 changes:

tl;dr:

BTF loader:

  - Support raw BTF as available in /sys/kernel/btf/vmlinux.

pahole:

  - When the sole argument passed isn't a file, take it as a class name:

     $ pahole sk_buff

  - Do not require a class name to operate without a file name.

      $ pahole          # is equivalent to:
      $ pahole vmlinux

  - Make --find_pointers_to consider unions:

      $ pahole --find_pointers_to ehci_qh

  - Make --contains and --find_pointers_to honour --unions

      $ pahole --unions --contains inet_sock

  - Add support for finding pointers to void:

     $ pahole --find_pointers_to void

  - Make --contains and --find_pointers_to to work with base types:

      $ pahole --find_pointers_to 'short unsigned int'

  - Make --contains look for more than just unions, structs:

      $ pahole --contains raw_spinlock_t

  - Consider unions when looking for classes containing some class:

      $ pahole --contains tpacket_req

  - Introduce --unions to consider just unions:

      $ pahole --unions --sizes
      $ pahole --unions --prefix tcp
      $ pahole --unions --nr_members

  - Fix -m/--nr_methods - Number of functions operating on a type pointer

      $ pahole --nr_methods

man-pages:

  - Add section about --hex + -E to locate offsets deep into sub structs.

  - Add more information about BTF.

  - Add some examples.

----------------------------------

I want the details:

btf loader:

  - Support raw BTF as available in /sys/kernel/btf/vmlinux

    Be it automatically when no -F option is passed and
    /sys/kernel/btf/vmlinux is available, or when /sys/kernel/btf/vmlinux is
    passed as the filename to the tool, i.e.:

      $ pahole -C list_head
      struct list_head {
            struct list_head *         next;                 /*     0     8 */
            struct list_head *         prev;                 /*     8     8 */

            /* size: 16, cachelines: 1, members: 2 */
            /* last cacheline: 16 bytes */
      };
      $ strace -e openat pahole -C list_head |& grep /sys/kernel/btf/
      openat(AT_FDCWD, "/sys/kernel/btf/vmlinux", O_RDONLY) = 3
      $
      $ pahole -C list_head /sys/kernel/btf/vmlinux
      struct list_head {
            struct list_head *         next;                 /*     0     8 */
            struct list_head *         prev;                 /*     8     8 */

            /* size: 16, cachelines: 1, members: 2 */
            /* last cacheline: 16 bytes */
      };
      $

    If one wants to grab the matching vmlinux to use its DWARF info instead,
    which is useful to compare the results with what we have from BTF, for
    instance, its just a matter of using '-F dwarf'.

    This in turn shows something that at first came as a surprise, but then
    has a simple explanation:

    For very common data structures, that will probably appear in all of the
    DWARF CUs (Compilation Units), like 'struct list_head', using '-F dwarf'
    is faster:

      $ perf stat -e cycles pahole -F btf -C list_head > /dev/null

       Performance counter stats for 'pahole -F btf -C list_head':

              45,722,518      cycles:u

             0.023717300 seconds time elapsed

             0.016474000 seconds user
             0.007212000 seconds sys

      $ perf stat -e cycles pahole -F dwarf -C list_head > /dev/null

       Performance counter stats for 'pahole -F dwarf -C list_head':

              14,170,321      cycles:u

             0.006668904 seconds time elapsed

             0.005562000 seconds user
             0.001109000 seconds sys

      $

    But for something that is more specific to a subsystem, the DWARF loader
    will have to process way more stuff till it gets to that struct:

      $ perf stat -e cycles pahole -F dwarf -C tcp_sock > /dev/null

       Performance counter stats for 'pahole -F dwarf -C tcp_sock':

          31,579,795,238      cycles:u

             8.332272930 seconds time elapsed

             8.032124000 seconds user
             0.286537000 seconds sys

      $

    While using the BTF loader the time should be constant, as it loads
    everything from /sys/kernel/btf/vmlinux:

      $ perf stat -e cycles pahole -F btf -C tcp_sock > /dev/null

       Performance counter stats for 'pahole -F btf -C tcp_sock':

              48,823,488      cycles:u

             0.024102760 seconds time elapsed

             0.012035000 seconds user
             0.012046000 seconds sys

      $

    Above I used '-F btf' just to show that it can be used, but its not
    really needed, i.e. those are equivalent:

      $ strace -e openat pahole -F btf -C list_head |& grep /sys/kernel/btf/vmlinux
      openat(AT_FDCWD, "/sys/kernel/btf/vmlinux", O_RDONLY) = 3
      $ strace -e openat pahole -C list_head |& grep /sys/kernel/btf/vmlinux
      openat(AT_FDCWD, "/sys/kernel/btf/vmlinux", O_RDONLY) = 3
      $

    The btf_raw__load() function that ends up being grafted into the
    preexisting btf_elf routines was based on libbpf's btf_load_raw().

pahole:

  - When the sole argument passed isn't a file, take it as a class name.

    With that it becomes as compact as it gets for kernel data structures,
    just state the name of the struct and it'll try to find that as a file,
    not being a file it'll use /sys/kernel/btf/vmlinux and the argument as a
    list of structs, i.e.:

      $ pahole skb_ext,list_head
      struct list_head {
            struct list_head *         next;                 /*     0     8 */
            struct list_head *         prev;                 /*     8     8 */

            /* size: 16, cachelines: 1, members: 2 */
            /* last cacheline: 16 bytes */
      };
      struct skb_ext {
            refcount_t                 refcnt;               /*     0     4 */
            u8                         offset[3];            /*     4     3 */
            u8                         chunks;               /*     7     1 */
            char                       data[];               /*     8     0 */

            /* size: 8, cachelines: 1, members: 4 */
            /* last cacheline: 8 bytes */
      };
      $ pahole hlist_node
      struct hlist_node {
            struct hlist_node *        next;                 /*     0     8 */
            struct hlist_node * *      pprev;                /*     8     8 */

            /* size: 16, cachelines: 1, members: 2 */
            /* last cacheline: 16 bytes */
      };
      $

    Of course -C continues to work:

      $ pahole -C inode | tail
            __u32                      i_fsnotify_mask;      /*   556     4 */
            struct fsnotify_mark_connector * i_fsnotify_marks; /*   560     8 */
            struct fscrypt_info *      i_crypt_info;         /*   568     8 */
            /* --- cacheline 9 boundary (576 bytes) --- */
            struct fsverity_info *     i_verity_info;        /*   576     8 */
            void *                     i_private;            /*   584     8 */

            /* size: 592, cachelines: 10, members: 53 */
            /* last cacheline: 16 bytes */
      };
      $

  - Add support for finding pointers to void, e.g.:

      $ pahole --find_pointers_to void --prefix tcp
      tcp_md5sig_pool: scratch
      $ pahole tcp_md5sig_pool
      struct tcp_md5sig_pool {
            struct ahash_request *     md5_req;              /*     0     8 */
            void *                     scratch;              /*     8     8 */

            /* size: 16, cachelines: 1, members: 2 */
            /* last cacheline: 16 bytes */
      };
      $

  - Make --contains and --find_pointers_to to work with base types

    I.e. with plain 'int', 'long', 'short int', etc:

      $ pahole --find_pointers_to 'short unsigned int'
      uv_hub_info_s: socket_to_node
      uv_hub_info_s: socket_to_pnode
      uv_hub_info_s: pnode_to_socket
      vc_data: vc_screenbuf
      vc_data: vc_translate
      filter_pred: ops
      ext4_sb_info: s_mb_offsets
      $ pahole ext4_sb_info | 'sort unsigned int'
      bash: sort unsigned int: command not found...
      ^[^C
      $
      $ pahole ext4_sb_info | grep 'sort unsigned int'
      $ pahole ext4_sb_info | grep 'short unsigned int'
            short unsigned int         s_mount_state;        /*   160     2 */
            short unsigned int         s_pad;                /*   162     2 */
            short unsigned int *       s_mb_offsets;         /*   664     8 */
      $ pahole --contains 'short unsigned int'
      apm_info
      desc_ptr
      thread_struct
      mpc_table
      mpc_intsrc
      fsnotify_mark_connector
      <SNIP>
      sock_fprog
      blk_mq_hw_ctx
      skb_shared_info
      $

  - Make --contains look for more than just unions, structs, look for
    typedefs, enums and types that descend from 'struct type':

    So now we can do more interesting queries, lets see, what are the data
    structures that embed a raw spinlock in the linux kernel?

      $ pahole --contains raw_spinlock_t
      task_struct
      rw_semaphore
      hrtimer_cpu_base
      prev_cputime
      percpu_counter
      ratelimit_state
      perf_event_context
      task_delay_info
      <SNIP>
      lpm_trie
      bpf_queue_stack
      $

    Look at the csets comments to see more examples.

  - Make --contains and --find_pointers_to honour --unions

    I.e. when looking for unions or structs that contains/embeds or looking
    for unions/structs that have pointers to a given type.

    E.g:

      $ pahole --contains inet_sock
      sctp_sock
      inet_connection_sock
      raw_sock
      udp_sock
      raw6_sock
      $ pahole --unions --contains inet_sock
      $

    We have structs embedding 'struct inet_sock', but no unions doing that.

  - Make --find_pointers_to consider unions

    I.e.:

      $ pahole --find_pointers_to ehci_qh
      ehci_hcd: qh_scan_next
      ehci_hcd: async
      ehci_hcd: dummy
      $

    Wasn't considering:

      $ pahole -C ehci_shadow
      union ehci_shadow {
            struct ehci_qh *           qh;                 /*     0     8 */
            struct ehci_itd *          itd;                /*     0     8 */
            struct ehci_sitd *         sitd;               /*     0     8 */
            struct ehci_fstn *         fstn;               /*     0     8 */
            __le32 *                   hw_next;            /*     0     8 */
            void *                     ptr;                /*     0     8 */
      };
      $

    Fix it:

      $ pahole --find_pointers_to ehci_qh
      ehci_hcd: qh_scan_next
      ehci_hcd: async
      ehci_hcd: dummy
      ehci_shadow: qh
      $

  - Consider unions when looking for classes containing some class:

    I.e.:

      $ pahole --contains tpacket_req
      tpacket_req_u
      $

    Wasn't working, but should be considered with --contains/-i:

      $ pahole -C tpacket_req_u
      union tpacket_req_u {
            struct tpacket_req         req;                /*     0    16 */
            struct tpacket_req3        req3;               /*     0    28 */
      };
      $

  - Introduce --unions to consider just unions

    Most filters can be used together with it, for instance to see the
    biggest unions in the kernel:

      $ pahole --unions --sizes | sort -k2 -nr | head
      thread_union            16384 0
      swap_header              4096 0
      fpregs_state             4096 0
      autofs_v5_packet_union    304 0
      autofs_packet_union       272 0
      pptp_ctrl_union           208 0
      acpi_parse_object         200 0
      acpi_descriptor           200 0
      bpf_attr                  120 0
      phy_configure_opts        112 0
      $

    Or just some unions that have some specific prefix:

      $ pahole --unions --prefix tcp
      union tcp_md5_addr {
            struct in_addr             a4;                 /*     0     4 */
            struct in6_addr            a6;                 /*     0    16 */
      };
      union tcp_word_hdr {
            struct tcphdr              hdr;                /*     0    20 */
            __be32                     words[5];           /*     0    20 */
      };
      union tcp_cc_info {
            struct tcpvegas_info       vegas;              /*     0    16 */
            struct tcp_dctcp_info      dctcp;              /*     0    16 */
            struct tcp_bbr_info        bbr;                /*     0    20 */
      };
      $

    What are the biggest unions in terms of number of members?

      $ pahole --unions --nr_members | sort -k2 -nr | head
      security_list_options 218
      aml_resource           36
      acpi_resource_data     29
      acpi_operand_object    26
      iwreq_data             18
      sctp_params            15
      ib_flow_spec           14
      ethtool_flow_union     14
      pptp_ctrl_union        13
      bpf_attr               12
      $

    If you want to script most of the queries can change the separator:

      $ pahole --unions --nr_members -t, | sort -t, -k2 -nr | head
      security_list_options,218
      aml_resource,36
      acpi_resource_data,29
      acpi_operand_object,26
      iwreq_data,18
      sctp_params,15
      ib_flow_spec,14
      ethtool_flow_union,14
      pptp_ctrl_union,13
      bpf_attr,12
      $

  - Fix -m/--nr_methods - Number of functions operating on a type pointer

    We had to use the same hack as in pfunct, as implemented in ccf3eebfcd9c
    ("btf_loader: Add support for BTF_KIND_FUNC"), will hide that 'struct
    ftype' (aka function prototype) indirection behind the parameter
    iterator (function__for_each_parameter).

    For now, here is the top 10 Linux kernel data structures in terms of
    number of functions receiving as one of its parameters a pointer to it,
    using /sys/kernel/btf/vmlinux to look at all the vmlinux types and
    functions (the ones visible in kallsyms, but with the parameters and its
    types):

      $ pahole -m | sort -k2 -nr | head
      device        955
      sock          568
      sk_buff       541
      task_struct   437
      inode         421
      pci_dev       390
      page          351
      net_device    347
      file          315
      net           312
      $
      $ pahole --help |& grep -- -m
        -m, --nr_methods           show number of methods
      $

  - Do not require a class name to operate without a file name

    Since we default to operating on the running kernel data structures, we
    should make the default to, with no options passed, to pretty print all
    the running kernel data structures, or do what was asked in terms of
    number of members, size of structs, etc, i.e.:

      # pahole --help |& head
      Usage: pahole [OPTION...] FILE

        -a, --anon_include         include anonymous classes
        -A, --nested_anon_include  include nested (inside other structs) anonymous
                                   classes
        -B, --bit_holes=NR_HOLES   Show only structs at least NR_HOLES bit holes
        -c, --cacheline_size=SIZE  set cacheline size to SIZE
            --classes_as_structs   Use 'struct' when printing classes
        -C, --class_name=CLASS_NAME   Show just this class
        -d, --recursive            recursive mode, affects several other flags
      #

    Continues working as before, but if you do:

      pahole

    It will work just as if you did:

      pahole vmlinux

    and that vmlinux file is the running kernel vmlinux.

    And since the default now is to read BTF info, then it will do all its
    operations on /sys/kernel/btf/vmlinux, when present, i.e. want to know
    what are the fattest data structures in the running kernel:

      # pahole -s | sort -k2 -nr | head
      cmp_data      290904  1
      dec_data      274520  1
      cpu_entry_area        217088  0
      pglist_data   172928  4
      saved_cmdlines_buffer 131104  1
      debug_store_buffers   131072  0
      hid_parser    110848  1
      hid_local     110608  0
      zonelist      81936   0
      e820_table    64004   0
      #

    How many data structures in the running kernel vmlinux area embbed
    'struct list_head'?

      # pahole -i list_head | wc -l
      260
      #

    Lets see some of those?

      # pahole -C fsnotify_event
      struct fsnotify_event {
            struct list_head           list;                 /*     0    16 */
            struct inode *             inode;                /*    16     8 */

            /* size: 24, cachelines: 1, members: 2 */
            /* last cacheline: 24 bytes */
      };
      # pahole -C audit_chunk
      struct audit_chunk {
            struct list_head           hash;                 /*     0    16 */
            long unsigned int          key;                  /*    16     8 */
            struct fsnotify_mark *     mark;                 /*    24     8 */
            struct list_head           trees;                /*    32    16 */
            int                        count;                /*    48     4 */

            /* XXX 4 bytes hole, try to pack */

            atomic_long_t              refs;                 /*    56     8 */
            /* --- cacheline 1 boundary (64 bytes) --- */
            struct callback_head       head;                 /*    64    16 */
            struct node                owners[];             /*    80     0 */

            /* size: 80, cachelines: 2, members: 8 */
            /* sum members: 76, holes: 1, sum holes: 4 */
            /* last cacheline: 16 bytes */
      };
      #