libbpf: Fix BTF-to-C converter's padding logic

Turns out that btf_dump API doesn't handle a bunch of tricky corner
cases, as reported by Per, and further discovered using his testing
Python script ([0]).

This patch revamps btf_dump's padding logic significantly, making it
more correct and also avoiding unnecessary explicit padding, where
compiler would pad naturally. This overall topic turned out to be very
tricky and subtle, there are lots of subtle corner cases. The comments
in the code tries to give some clues, but comments themselves are
supposed to be paired with good understanding of C alignment and padding
rules. Plus some experimentation to figure out subtle things like
whether `long :0;` means that struct is now forced to be long-aligned
(no, it's not, turns out).

Anyways, Per's script, while not completely correct in some known
situations, doesn't show any obvious cases where this logic breaks, so
this is a nice improvement over the previous state of this logic.

Some selftests had to be adjusted to accommodate better use of natural
alignment rules, eliminating some unnecessary padding, or changing it to
`type: 0;` alignment markers.

Note also that for when we are in between bitfields, we emit explicit
bit size, while otherwise we use `: 0`, this feels much more natural in
practice.

Next patch will add few more test cases, found through randomized Per's
script.

[0] https://lore.kernel.org/bpf/[e-mail]/

Reported-by: Per Sundström XP <[e-mail]>
Signed-off-by: Andrii Nakryiko <[e-mail]>
Signed-off-by: Daniel Borkmann <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

tools/lib/bpf/btf_dump.c

···
 	}
 }
 
+static int btf_natural_align_of(const struct btf *btf, __u32 id)
+{
+	const struct btf_type *t = btf__type_by_id(btf, id);
+	int i, align, vlen;
+	const struct btf_member *m;
+
+	if (!btf_is_composite(t))
+		return btf__align_of(btf, id);
+
+	align = 1;
+	m = btf_members(t);
+	vlen = btf_vlen(t);
+	for (i = 0; i < vlen; i++, m++) {
+		align = max(align, btf__align_of(btf, m->type));
+	}
+
+	return align;
+}
+
 static bool btf_is_struct_packed(const struct btf *btf, __u32 id,
 				 const struct btf_type *t)
 {
···
 	int align, i, bit_sz;
 	__u16 vlen;
 
-	align = btf__align_of(btf, id);
-	/* size of a non-packed struct has to be a multiple of its alignment*/
-	if (align && t->size % align)
+	align = btf_natural_align_of(btf, id);
+	/* size of a non-packed struct has to be a multiple of its alignment */
+	if (align && (t->size % align) != 0)
 		return true;
 
 	m = btf_members(t);
 	vlen = btf_vlen(t);
 	/* all non-bitfield fields have to be naturally aligned */
 	for (i = 0; i < vlen; i++, m++) {
-		align = btf__align_of(btf, m->type);
+		align = btf_natural_align_of(btf, m->type);
 		bit_sz = btf_member_bitfield_size(t, i);
 		if (align && bit_sz == 0 && m->offset % (8 * align) != 0)
 			return true;
···
 	return false;
 }
 
-static int chip_away_bits(int total, int at_most)
-{
-	return total % at_most ? : at_most;
-}
-
 static void btf_dump_emit_bit_padding(const struct btf_dump *d,
-				      int cur_off, int m_off, int m_bit_sz,
-				      int align, int lvl)
+				      int cur_off, int next_off, int next_align,
+				      bool in_bitfield, int lvl)
 {
-	int off_diff = m_off - cur_off;
-	int ptr_bits = d->ptr_sz * 8;
+	const struct {
+		const char *name;
+		int bits;
+	} pads[] = {
+		{"long", d->ptr_sz * 8}, {"int", 32}, {"short", 16}, {"char", 8}
+	};
+	int new_off, pad_bits, bits, i;
+	const char *pad_type;
 
-	if (off_diff <= 0)
-		/* no gap */
-		return;
-	if (m_bit_sz == 0 && off_diff < align * 8)
-		/* natural padding will take care of a gap */
-		return;
+	if (cur_off >= next_off)
+		return; /* no gap */
 
-	while (off_diff > 0) {
-		const char *pad_type;
-		int pad_bits;
+	/* For filling out padding we want to take advantage of
+	 * natural alignment rules to minimize unnecessary explicit
+	 * padding. First, we find the largest type (among long, int,
+	 * short, or char) that can be used to force naturally aligned
+	 * boundary. Once determined, we'll use such type to fill in
+	 * the remaining padding gap. In some cases we can rely on
+	 * compiler filling some gaps, but sometimes we need to force
+	 * alignment to close natural alignment with markers like
+	 * `long: 0` (this is always the case for bitfields).  Note
+	 * that even if struct itself has, let's say 4-byte alignment
+	 * (i.e., it only uses up to int-aligned types), using `long:
+	 * X;` explicit padding doesn't actually change struct's
+	 * overall alignment requirements, but compiler does take into
+	 * account that type's (long, in this example) natural
+	 * alignment requirements when adding implicit padding. We use
+	 * this fact heavily and don't worry about ruining correct
+	 * struct alignment requirement.
+	 */
+	for (i = 0; i < ARRAY_SIZE(pads); i++) {
+		pad_bits = pads[i].bits;
+		pad_type = pads[i].name;
 
-		if (ptr_bits > 32 && off_diff > 32) {
-			pad_type = "long";
-			pad_bits = chip_away_bits(off_diff, ptr_bits);
-		} else if (off_diff > 16) {
-			pad_type = "int";
-			pad_bits = chip_away_bits(off_diff, 32);
-		} else if (off_diff > 8) {
-			pad_type = "short";
-			pad_bits = chip_away_bits(off_diff, 16);
-		} else {
-			pad_type = "char";
-			pad_bits = chip_away_bits(off_diff, 8);
+		new_off = roundup(cur_off, pad_bits);
+		if (new_off <= next_off)
+			break;
+	}
+
+	if (new_off > cur_off && new_off <= next_off) {
+		/* We need explicit `<type>: 0` aligning mark if next
+		 * field is right on alignment offset and its
+		 * alignment requirement is less strict than <type>'s
+		 * alignment (so compiler won't naturally align to the
+		 * offset we expect), or if subsequent `<type>: X`,
+		 * will actually completely fit in the remaining hole,
+		 * making compiler basically ignore `<type>: X`
+		 * completely.
+		 */
+		if (in_bitfield ||
+		    (new_off == next_off && roundup(cur_off, next_align * 8) != new_off) ||
+		    (new_off != next_off && next_off - new_off <= new_off - cur_off))
+			/* but for bitfields we'll emit explicit bit count */
+			btf_dump_printf(d, "\n%s%s: %d;", pfx(lvl), pad_type,
+					in_bitfield ? new_off - cur_off : 0);
+		cur_off = new_off;
+	}
+
+	/* Now we know we start at naturally aligned offset for a chosen
+	 * padding type (long, int, short, or char), and so the rest is just
+	 * a straightforward filling of remaining padding gap with full
+	 * `<type>: sizeof(<type>);` markers, except for the last one, which
+	 * might need smaller than sizeof(<type>) padding.
+	 */
+	while (cur_off != next_off) {
+		bits = min(next_off - cur_off, pad_bits);
+		if (bits == pad_bits) {
+			btf_dump_printf(d, "\n%s%s: %d;", pfx(lvl), pad_type, pad_bits);
+			cur_off += bits;
+			continue;
 		}
-		btf_dump_printf(d, "\n%s%s: %d;", pfx(lvl), pad_type, pad_bits);
-		off_diff -= pad_bits;
+		/* For the remainder padding that doesn't cover entire
+		 * pad_type bit length, we pick the smallest necessary type.
+		 * This is pure aesthetics, we could have just used `long`,
+		 * but having smallest necessary one communicates better the
+		 * scale of the padding gap.
+		 */
+		for (i = ARRAY_SIZE(pads) - 1; i >= 0; i--) {
+			pad_type = pads[i].name;
+			pad_bits = pads[i].bits;
+			if (pad_bits < bits)
+				continue;
+
+			btf_dump_printf(d, "\n%s%s: %d;", pfx(lvl), pad_type, bits);
+			cur_off += bits;
+			break;
+		}
 	}
 }
 
···
 {
 	const struct btf_member *m = btf_members(t);
 	bool is_struct = btf_is_struct(t);
-	int align, i, packed, off = 0;
+	bool packed, prev_bitfield = false;
+	int align, i, off = 0;
 	__u16 vlen = btf_vlen(t);
 
+	align = btf__align_of(d->btf, id);
 	packed = is_struct ? btf_is_struct_packed(d->btf, id, t) : 0;
 
 	btf_dump_printf(d, "%s%s%s {",
···
 
 	for (i = 0; i < vlen; i++, m++) {
 		const char *fname;
-		int m_off, m_sz;
+		int m_off, m_sz, m_align;
+		bool in_bitfield;
 
 		fname = btf_name_of(d, m->name_off);
 		m_sz = btf_member_bitfield_size(t, i);
 		m_off = btf_member_bit_offset(t, i);
-		align = packed ? 1 : btf__align_of(d->btf, m->type);
+		m_align = packed ? 1 : btf__align_of(d->btf, m->type);
 
-		btf_dump_emit_bit_padding(d, off, m_off, m_sz, align, lvl + 1);
+		in_bitfield = prev_bitfield && m_sz != 0;
+
+		btf_dump_emit_bit_padding(d, off, m_off, m_align, in_bitfield, lvl + 1);
 		btf_dump_printf(d, "\n%s", pfx(lvl + 1));
 		btf_dump_emit_type_decl(d, m->type, fname, lvl + 1);
 
 		if (m_sz) {
 			btf_dump_printf(d, ": %d", m_sz);
 			off = m_off + m_sz;
+			prev_bitfield = true;
 		} else {
 			m_sz = max((__s64)0, btf__resolve_size(d->btf, m->type));
 			off = m_off + m_sz * 8;
+			prev_bitfield = false;
 		}
+
 		btf_dump_printf(d, ";");
 	}
 
 	/* pad at the end, if necessary */
-	if (is_struct) {
-		align = packed ? 1 : btf__align_of(d->btf, id);
-		btf_dump_emit_bit_padding(d, off, t->size * 8, 0, align,
-					  lvl + 1);
-	}
+	if (is_struct)
+		btf_dump_emit_bit_padding(d, off, t->size * 8, align, false, lvl + 1);
 
 	/*
 	 * Keep `struct empty {}` on a single line,

tools/testing/selftests/bpf/progs/btf_dump_test_case_bitfields.c

···
  */
 /* ------ END-EXPECTED-OUTPUT ------ */
 struct bitfield_mixed_with_others {
-	long: 4; /* char is enough as a backing field */
+	char: 4; /* char is enough as a backing field */
 	int a: 4;
 	/* 8-bit implicit padding */
 	short b; /* combined with previous bitfield */

tools/testing/selftests/bpf/progs/btf_dump_test_case_padding.c

···
 /*
  *struct padded_explicitly {
  *	int a;
- *	int: 32;
+ *	long: 0;
  *	int b;
  *};
  *
···
 
 struct padded_explicitly {
 	int a;
-	int: 1; /* algo will explicitly pad with full 32 bits here */
+	int: 1; /* algo will emit aligning `long: 0;` here */
 	int b;
 };
 
 /* ----- START-EXPECTED-OUTPUT ----- */
-/*
- *struct padded_a_lot {
- *	int a;
- *	long: 32;
- *	long: 64;
- *	long: 64;
- *	int b;
- *};
- *
- */
-/* ------ END-EXPECTED-OUTPUT ------ */
-
 struct padded_a_lot {
 	int a;
-	/* 32 bit of implicit padding here, which algo will make explicit */
 	long: 64;
 	long: 64;
 	int b;
 };
+
+/* ------ END-EXPECTED-OUTPUT ------ */
 
 /* ----- START-EXPECTED-OUTPUT ----- */
 /*
  *struct padded_cache_line {
  *	int a;
- *	long: 32;
  *	long: 64;
  *	long: 64;
  *	long: 64;
  *	int b;
- *	long: 32;
  *	long: 64;
  *	long: 64;
  *	long: 64;
···
  *struct zone {
  *	int a;
  *	short b;
- *	short: 16;
+ *	long: 0;
  *	struct zone_padding __pad__;
  *};
  *
···
 	long: 64;
 };
 
+struct padding_weird_1 {
+	int a;
+	long: 64;
+	short: 16;
+	short b;
+};
+
+/* ------ END-EXPECTED-OUTPUT ------ */
+
+/* ----- START-EXPECTED-OUTPUT ----- */
+/*
+ *struct padding_weird_2 {
+ *	long: 56;
+ *	char a;
+ *	long: 56;
+ *	char b;
+ *	char: 8;
+ *};
+ *
+ */
+/* ------ END-EXPECTED-OUTPUT ------ */
+struct padding_weird_2 {
+	int: 32;	/* these paddings will be collapsed into `long: 56;` */
+	short: 16;
+	char: 8;
+	char a;
+	int: 32;	/* these paddings will be collapsed into `long: 56;` */
+	short: 16;
+	char: 8;
+	char b;
+	char: 8;
+};
+
 /* ------ END-EXPECTED-OUTPUT ------ */
 
 int f(struct {
···
 	struct padded_cache_line _4;
 	struct zone _5;
 	struct padding_wo_named_members _6;
+	struct padding_weird_1 _7;
+	struct padding_weird_2 _8;
 } *_)
 {
 	return 0;