Re: [PATCH v3 1/4] btf: add support for ints larger than 128 bits
From: Andrii Nakryiko
Date: Wed Jan 06 2021 - 00:17:39 EST
On Tue, Jan 5, 2021 at 9:10 PM Andrii Nakryiko
<andrii.nakryiko@xxxxxxxxx> wrote:
>
> On Tue, Jan 5, 2021 at 6:45 AM Sean Young <sean@xxxxxxxx> wrote:
> >
> > clang supports arbitrary length ints using the _ExtInt extension. This
> > can be useful to hold very large values, e.g. 256 bit or 512 bit types.
> >
> > Larger types (e.g. 1024 bits) are possible but I am unaware of a use
> > case for these.
> >
> > This requires the _ExtInt extension enabled in clang, which is under
> > review.
> >
> > Link: https://clang.llvm.org/docs/LanguageExtensions.html#extended-integer-types
> > Link: https://reviews.llvm.org/D93103
> >
> > Signed-off-by: Sean Young <sean@xxxxxxxx>
> > ---
> > Documentation/bpf/btf.rst | 4 +--
> > include/uapi/linux/btf.h | 2 +-
> > kernel/bpf/btf.c | 54 ++++++++++++++++++++++++++++------
> > tools/include/uapi/linux/btf.h | 2 +-
> > 4 files changed, 49 insertions(+), 13 deletions(-)
> >
> > diff --git a/Documentation/bpf/btf.rst b/Documentation/bpf/btf.rst
> > index 44dc789de2b4..784f1743dbc7 100644
> > --- a/Documentation/bpf/btf.rst
> > +++ b/Documentation/bpf/btf.rst
> > @@ -132,7 +132,7 @@ The following sections detail encoding of each kind.
> >
> > #define BTF_INT_ENCODING(VAL) (((VAL) & 0x0f000000) >> 24)
> > #define BTF_INT_OFFSET(VAL) (((VAL) & 0x00ff0000) >> 16)
> > - #define BTF_INT_BITS(VAL) ((VAL) & 0x000000ff)
> > + #define BTF_INT_BITS(VAL) ((VAL) & 0x000003ff)
> >
> > The ``BTF_INT_ENCODING`` has the following attributes::
> >
> > @@ -147,7 +147,7 @@ pretty print. At most one encoding can be specified for the int type.
> > The ``BTF_INT_BITS()`` specifies the number of actual bits held by this int
> > type. For example, a 4-bit bitfield encodes ``BTF_INT_BITS()`` equals to 4.
> > The ``btf_type.size * 8`` must be equal to or greater than ``BTF_INT_BITS()``
> > -for the type. The maximum value of ``BTF_INT_BITS()`` is 128.
> > +for the type. The maximum value of ``BTF_INT_BITS()`` is 512.
> >
> > The ``BTF_INT_OFFSET()`` specifies the starting bit offset to calculate values
> > for this int. For example, a bitfield struct member has:
> > diff --git a/include/uapi/linux/btf.h b/include/uapi/linux/btf.h
> > index 5a667107ad2c..1696fd02b302 100644
> > --- a/include/uapi/linux/btf.h
> > +++ b/include/uapi/linux/btf.h
> > @@ -84,7 +84,7 @@ struct btf_type {
> > */
> > #define BTF_INT_ENCODING(VAL) (((VAL) & 0x0f000000) >> 24)
> > #define BTF_INT_OFFSET(VAL) (((VAL) & 0x00ff0000) >> 16)
> > -#define BTF_INT_BITS(VAL) ((VAL) & 0x000000ff)
> > +#define BTF_INT_BITS(VAL) ((VAL) & 0x000003ff)
> >
> > /* Attributes stored in the BTF_INT_ENCODING */
> > #define BTF_INT_SIGNED (1 << 0)
> > diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> > index 8d6bdb4f4d61..44bc17207e9b 100644
> > --- a/kernel/bpf/btf.c
> > +++ b/kernel/bpf/btf.c
> > @@ -166,7 +166,8 @@
> > *
> > */
> >
> > -#define BITS_PER_U128 (sizeof(u64) * BITS_PER_BYTE * 2)
> > +#define BITS_PER_U128 128
> > +#define BITS_PER_U512 512
> > #define BITS_PER_BYTE_MASK (BITS_PER_BYTE - 1)
> > #define BITS_PER_BYTE_MASKED(bits) ((bits) & BITS_PER_BYTE_MASK)
> > #define BITS_ROUNDDOWN_BYTES(bits) ((bits) >> 3)
> > @@ -1907,9 +1908,9 @@ static int btf_int_check_member(struct btf_verifier_env *env,
> > nr_copy_bits = BTF_INT_BITS(int_data) +
> > BITS_PER_BYTE_MASKED(struct_bits_off);
> >
> > - if (nr_copy_bits > BITS_PER_U128) {
> > + if (nr_copy_bits > BITS_PER_U512) {
> > btf_verifier_log_member(env, struct_type, member,
> > - "nr_copy_bits exceeds 128");
> > + "nr_copy_bits exceeds 512");
> > return -EINVAL;
> > }
> >
> > @@ -1963,9 +1964,9 @@ static int btf_int_check_kflag_member(struct btf_verifier_env *env,
> >
> > bytes_offset = BITS_ROUNDDOWN_BYTES(struct_bits_off);
> > nr_copy_bits = nr_bits + BITS_PER_BYTE_MASKED(struct_bits_off);
> > - if (nr_copy_bits > BITS_PER_U128) {
> > + if (nr_copy_bits > BITS_PER_U512) {
> > btf_verifier_log_member(env, struct_type, member,
> > - "nr_copy_bits exceeds 128");
> > + "nr_copy_bits exceeds 512");
> > return -EINVAL;
> > }
> >
> > @@ -2012,9 +2013,9 @@ static s32 btf_int_check_meta(struct btf_verifier_env *env,
> >
> > nr_bits = BTF_INT_BITS(int_data) + BTF_INT_OFFSET(int_data);
> >
> > - if (nr_bits > BITS_PER_U128) {
> > - btf_verifier_log_type(env, t, "nr_bits exceeds %zu",
> > - BITS_PER_U128);
> > + if (nr_bits > BITS_PER_U512) {
> > + btf_verifier_log_type(env, t, "nr_bits exceeds %u",
> > + BITS_PER_U512);
> > return -EINVAL;
> > }
> >
> > @@ -2080,6 +2081,37 @@ static void btf_int128_print(struct btf_show *show, void *data)
> > lower_num);
> > }
> >
> > +static void btf_bigint_print(struct btf_show *show, void *data, u16 nr_bits)
> > +{
> > + /* data points to 256 or 512 bit int type */
> > + char buf[129];
> > + int last_u64 = nr_bits / 64 - 1;
> > + bool seen_nonzero = false;
> > + int i;
> > +
> > + for (i = 0; i <= last_u64; i++) {
> > +#ifdef __BIG_ENDIAN_BITFIELD
> > + u64 v = ((u64 *)data)[i];
> > +#else
> > + u64 v = ((u64 *)data)[last_u64 - i];
> > +#endif
to avoid duplicating this #ifdef with my suggestion, you can do something like
#ifdef __BIG_ENDIAN_BITFIELD
u64 *v = (u64 *)data;
int step = 1;
#else
u64 *v = (u64 *)data + last_u64;
int step = -1;
#endif
and then just `v += step;` everywhere
> > + if (!seen_nonzero) {
> > + if (!v && i != last_u64)
> > + continue;
> > +
> > + snprintf(buf, sizeof(buf), "%llx", v);
> > +
> > + seen_nonzero = true;
> > + } else {
> > + size_t off = strlen(buf);
>
> this is wasteful, snprintf() returns number of characters printed, so
> you can maintain offset properly
>
> > +
> > + snprintf(buf + off, sizeof(buf) - off, "%016llx", v);
> > + }
> > + }
> > +
> > + btf_show_type_value(show, "0x%s", buf);
> > +}
>
> seen_nonzero is a bit convoluted, two simple loops might be more
> straightforward:
>
> u64 v;
> int off;
>
> /* find first non-zero u64 (or stop on the last one regardless) */
> for (i = 0; i < last_u64; i++) {
> v = ...;
> if (!v)
> continue;
> }
> /* print non-zero or zero, but last u64 */
> off = snprintf(buf, sizeof(buf), "%llx", v);
> /* print the rest with zero padding */
> for (i++; i <= last_u64; i++) {
> v = ...;
> off += snprintf(buf + off, sizeof(buf) - off, "%016llx", v);
> }
>
> > +
> > static void btf_int128_shift(u64 *print_num, u16 left_shift_bits,
> > u16 right_shift_bits)
> > {
> > @@ -2172,7 +2204,7 @@ static void btf_int_show(const struct btf *btf, const struct btf_type *t,
> > u32 int_data = btf_type_int(t);
> > u8 encoding = BTF_INT_ENCODING(int_data);
> > bool sign = encoding & BTF_INT_SIGNED;
> > - u8 nr_bits = BTF_INT_BITS(int_data);
> > + u16 nr_bits = BTF_INT_BITS(int_data);
> > void *safe_data;
> >
> > safe_data = btf_show_start_type(show, t, type_id, data);
> > @@ -2186,6 +2218,10 @@ static void btf_int_show(const struct btf *btf, const struct btf_type *t,
> > }
> >
> > switch (nr_bits) {
> > + case 512:
> > + case 256:
> > + btf_bigint_print(show, safe_data, nr_bits);
> > + break;
> > case 128:
> > btf_int128_print(show, safe_data);
>
> btf_bigint_print() supersedes btf_int128_print(), why maintain both?
>
> > break;
> > diff --git a/tools/include/uapi/linux/btf.h b/tools/include/uapi/linux/btf.h
> > index 5a667107ad2c..1696fd02b302 100644
> > --- a/tools/include/uapi/linux/btf.h
> > +++ b/tools/include/uapi/linux/btf.h
> > @@ -84,7 +84,7 @@ struct btf_type {
> > */
> > #define BTF_INT_ENCODING(VAL) (((VAL) & 0x0f000000) >> 24)
> > #define BTF_INT_OFFSET(VAL) (((VAL) & 0x00ff0000) >> 16)
> > -#define BTF_INT_BITS(VAL) ((VAL) & 0x000000ff)
> > +#define BTF_INT_BITS(VAL) ((VAL) & 0x000003ff)
> >
> > /* Attributes stored in the BTF_INT_ENCODING */
> > #define BTF_INT_SIGNED (1 << 0)
> > --
> > 2.29.2
> >