Re: [PATCH bpf-next v4 4/8] libbpf: Support BTF.ext loading and output in either endianness

On Fri, Aug 30, 2024 at 12:30 AM Tony Ambardar tony.ambardar@gmail.com wrote:

Support for handling BTF data of either endianness was added in [1], but did not include BTF.ext data for lack of use cases. Later, support for static linking [2] provided a use case, but this feature and later ones were restricted to native-endian usage.

Add support for BTF.ext handling in either endianness. Convert BTF.ext data to native endianness when read into memory for further processing, and support raw data access that restores the original byte-order for output. Add internal header functions for byte-swapping func, line, and core info records.

Add new API functions btf_ext__endianness() and btf_ext__set_endianness() for query and setting byte-order, as already exist for BTF data.

[1] 3289959b97ca ("libbpf: Support BTF loading and raw data output in both endianness") [2] 8fd27bf69b86 ("libbpf: Add BPF static linker BTF and BTF.ext support")

Signed-off-by: Tony Ambardar tony.ambardar@gmail.com

tools/lib/bpf/btf.c | 192 +++++++++++++++++++++++++++++--- tools/lib/bpf/btf.h | 3 + tools/lib/bpf/libbpf.map | 2 + tools/lib/bpf/libbpf_internal.h | 33 ++++++ 4 files changed, 214 insertions(+), 16 deletions(-)

diff --git a/tools/lib/bpf/btf.c b/tools/lib/bpf/btf.c index f5081de86ee0..064cfe126c09 100644 --- a/tools/lib/bpf/btf.c +++ b/tools/lib/bpf/btf.c @@ -3022,25 +3022,102 @@ static int btf_ext_setup_core_relos(struct btf_ext *btf_ext) return btf_ext_setup_info(btf_ext, &param); }

-static int btf_ext_parse_hdr(__u8 *data, __u32 data_size) +/* Swap byte-order of BTF.ext header with any endianness */ +static void btf_ext_bswap_hdr(struct btf_ext *btf_ext, __u32 hdr_len) {

•   const struct btf_ext_header *hdr = (struct btf_ext_header *)data;
  

•   struct btf_ext_header *h = btf_ext->hdr;
  
  

•   if (data_size < offsetofend(struct btf_ext_header, hdr_len) ||
  

•       data_size < hdr->hdr_len) {
  

•           pr_debug("BTF.ext header not found\n");
  

•   h->magic = bswap_16(h->magic);
  

•   h->hdr_len = bswap_32(h->hdr_len);
  

•   h->func_info_off = bswap_32(h->func_info_off);
  

•   h->func_info_len = bswap_32(h->func_info_len);
  

•   h->line_info_off = bswap_32(h->line_info_off);
  

•   h->line_info_len = bswap_32(h->line_info_len);
  

•   if (hdr_len < offsetofend(struct btf_ext_header, core_relo_len))
  

•           return;
  

•   h->core_relo_off = bswap_32(h->core_relo_off);
  

•   h->core_relo_len = bswap_32(h->core_relo_len);
  

+}

+/* Swap byte-order of a generic info subsection */ +static void info_subsec_bswap(const struct btf_ext_header *hdr, bool native,

•                         __u32 off, __u32 len, anon_info_bswap_fn_t bswap)
  

ok, so I'm not a fan of this bswap callback, tbh. Also, we don't really enforce that each kind of record has exact size we expect (i.e., bpf_line_info_min and bpf_func_info_min shouldn't be "min" for byte-swapped case, it should be exact).

How about this slight modification: split byte swapping of sections/subsection metadata, so we adjust record size, sec_name_off and num_info separately from adjusting each record.

Once this swapping is done we:

a) validate record size for each section is expected (according to its type, of course) b) we can then use for_each_btf_ext_sec() and for_each_btf_ext_rec() macro (which assume proper in-memory metadata byte order) and then hard-code swapping of each record fields

No callbacks.

This has also a benefit of not needing this annoying "bool native" flag when producing raw bytes. We just ensure proper order of operation:

a) swap records b) swap metadata (so just mirrored order from initialization)

WDYT?

pw-bot: cr

+{

•   __u32 left, i, *rs, rec_size, num_info;
  

•   struct btf_ext_info_sec *si;
  

•   void *p;
  

•   if (len == 0)
  

•           return;
  

•   rs = (void *)hdr + hdr->hdr_len + off;  /* record size */
  

•   si = (void *)rs + sizeof(__u32);        /* sec info #1 */
  

•   rec_size = native ? *rs : bswap_32(*rs);
  

•   *rs = bswap_32(*rs);
  

•   left = len - sizeof(__u32);
  

•   while (left > 0) {
  

•           num_info = native ? si->num_info : bswap_32(si->num_info);
  

•           si->sec_name_off = bswap_32(si->sec_name_off);
  

•           si->num_info = bswap_32(si->num_info);
  

•           left -= offsetof(struct btf_ext_info_sec, data);
  

•           p = si->data;
  

•           for (i = 0; i < num_info; i++)  /* list of records */
  

•                   p += bswap(p);
  

•           si = p;
  

•           left -=  rec_size * num_info;
  

nit: extra space here

•   }
  

+}

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