Add a libbpf dumper function that supports dumping a representation of data passed in using the BTF id associated with the data in a manner similar to the bpf_snprintf_btf helper.
Default output format is identical to that dumped by bpf_snprintf_btf() (bar using tabs instead of spaces for indentation, but the indent string can be customized also); for example, a "struct sk_buff" representation would look like this:
(struct sk_buff){ (union){ (struct){ .next = (struct sk_buff *)0xffffffffffffffff, .prev = (struct sk_buff *)0xffffffffffffffff, (union){ .dev = (struct net_device *)0xffffffffffffffff, .dev_scratch = (long unsigned int)18446744073709551615, }, }, ...
Patch 1 implements the dump functionality in a manner similar to that in kernel/bpf/btf.c, but with a view to fitting into libbpf more naturally. For example, rather than using flags, boolean dump options are used to control output. In addition, rather than combining checks for display (such as is this field zero?) and actual display - as is done for the kernel code - the code is organized to separate zero and overflow checks from type display.
Patch 2 consists of selftests that utilize a dump printf function to snprintf the dump output to a string for comparison with expected output. Tests deliberately mirror those in snprintf_btf helper test to keep output consistent, but also cover overflow handling, var/section display.
Changes since v3 [1] - Retained separation of emitting of type name cast prefixing type values from existing functionality such as btf_dump_emit_type_chain() since initial code-shared version had so many exceptions it became hard to read. For example, we don't emit a type name if the type to be displayed is an array member, we also always emit "forward" definitions for structs/unions that aren't really forward definitions (we just want a "struct foo" output for "(struct foo){.bar = ...". We also always ignore modifiers const/volatile/restrict as they clutter output when emitting large types. - Added configurable 4-char indent string option; defaults to tab (Andrii) - Added support for BTF_KIND_FLOAT and associated tests (Andrii) - Added support for BTF_KIND_FUNC_PROTO function pointers to improve output of "ops" structures; for example:
(struct file_operations){ .owner = (struct module *)0xffffffffffffffff, .llseek = (loff_t(*)(struct file *, loff_t, int))0xffffffffffffffff, ... Added associated test also (Andrii) - Added handling for enum bitfields and associated test (Andrii) - Allocation of "struct btf_dump_data" done on-demand (Andrii) - Removed ".field = " output from function emitting type name and into caller (Andrii) - Removed BTF_INT_OFFSET() support (Andrii) - Use libbpf_err() to set errno for error cases (Andrii) - btf_dump_dump_type_data() returns size written, which is used when returning successfully from btf_dump__dump_type_data() (Andrii)
Changes since v2 [2]
- Renamed function to btf_dump__dump_type_data, reorganized arguments such that opts are last (Andrii) - Modified code to separate questions about display such as have we overflowed?/is this field zero? from actual display of typed data, such that we ask those questions separately from the code that actually displays typed data (Andrii) - Reworked code to handle overflow - where we do not provide enough data for the type we wish to display - by returning -E2BIG and attempting to present as much data as possible. Such a mode of operation allows for tracers which retrieve partial data (such as first 1024 bytes of a "struct task_struct" say), and want to display that partial data, while also knowing that it is not the full type. Such tracers can then denote this (perhaps via "..." or similar). - Explored reusing existing type emit functions, such as passing in a type id stack with a single type id to btf_dump_emit_type_chain() to support the display of typed data where a "cast" is prepended to the data to denote its type; "(int)1", "(struct foo){", etc. However the task of emitting a ".field_name = (typecast)" did not match well with model of walking the stack to display innermost types first and made the resultant code harder to read. Added a dedicated btf_dump_emit_type_name() function instead which is only ~70 lines (Andrii) - Various cleanups around bitfield macros, unneeded member iteration macros, avoiding compiler complaints when displaying int da ta by casting to long long, etc (Andrii) - Use DECLARE_LIBBPF_OPTS() in defining opts for tests (Andrii) - Added more type tests, overflow tests, var tests and section tests.
Changes since RFC [3]
- The initial approach explored was to share the kernel code with libbpf using #defines to paper over the different needs; however it makes more sense to try and fit in with libbpf code style for maintenance. A comment in the code points at the implementation in kernel/bpf/btf.c and notes that any issues found in it should be fixed there or vice versa; mirroring the tests should help with this also (Andrii)
[1] https://lore.kernel.org/bpf/1622131170-8260-1-git-send-email-alan.maguire@or... [2] https://lore.kernel.org/bpf/1610921764-7526-1-git-send-email-alan.maguire@or... [3] https://lore.kernel.org/bpf/1610386373-24162-1-git-send-email-alan.maguire@o...
Alan Maguire (2): libbpf: BTF dumper support for typed data selftests/bpf: add dump type data tests to btf dump tests
tools/lib/bpf/btf.h | 22 + tools/lib/bpf/btf_dump.c | 1008 ++++++++++++++++++++- tools/lib/bpf/libbpf.map | 1 + tools/testing/selftests/bpf/prog_tests/btf_dump.c | 638 +++++++++++++ 4 files changed, 1667 insertions(+), 2 deletions(-)