bpf: Introduce "volatile compare" macros

Compilers optimize conditional operators at will, but often bpf programmers
want to force compilers to keep the same operator in asm as it's written in C.
Introduce bpf_cmp_likely/unlikely(var1, conditional_op, var2) macros that can be used as:

-               if (seen >= 1000)
+               if (bpf_cmp_unlikely(seen, >=, 1000))

The macros take advantage of BPF assembly that is C like.

The macros check the sign of variable 'seen' and emits either
signed or unsigned compare.

For example:
int a;
bpf_cmp_unlikely(a, >, 0) will be translated to 'if rX s> 0 goto' in BPF assembly.

unsigned int a;
bpf_cmp_unlikely(a, >, 0) will be translated to 'if rX > 0 goto' in BPF assembly.

C type conversions coupled with comparison operator are tricky.
  int i = -1;
  unsigned int j = 1;
  if (i < j) // this is false.

  long i = -1;
  unsigned int j = 1;
  if (i < j) // this is true.

Make sure BPF program is compiled with -Wsign-compare then the macros will catch
the mistake.

The macros check LHS (left hand side) only to figure out the sign of compare.

'if 0 < rX goto' is not allowed in the assembly, so the users
have to use a variable on LHS anyway.

The patch updates few tests to demonstrate the use of the macros.

The macro allows to use BPF_JSET in C code, since LLVM doesn't generate it at
present. For example:

if (i & j) compiles into r0 &= r1; if r0 == 0 goto

while

if (bpf_cmp_unlikely(i, &, j)) compiles into if r0 & r1 goto

Note that the macros has to be careful with RHS assembly predicate.
Since:
u64 __rhs = 1ull << 42;
asm goto("if r0 < %[rhs] goto +1" :: [rhs] "ri" (__rhs));
LLVM will silently truncate 64-bit constant into s32 imm.

Note that [lhs] "r"((short)LHS) the type cast is a workaround for LLVM issue.
When LHS is exactly 32-bit LLVM emits redundant <<=32, >>=32 to zero upper 32-bits.
When LHS is 64 or 16 or 8-bit variable there are no shifts.
When LHS is 32-bit the (u64) cast doesn't help. Hence use (short) cast.
It does _not_ truncate the variable before it's assigned to a register.

Traditional likely()/unlikely() macros that use __builtin_expect(!!(x), 1 or 0)
have no effect on these macros, hence macros implement the logic manually.
bpf_cmp_unlikely() macro preserves compare operator as-is while
bpf_cmp_likely() macro flips the compare.

Consider two cases:
A.
  for() {
    if (foo >= 10) {
      bar += foo;
    }
    other code;
  }

B.
  for() {
    if (foo >= 10)
       break;
    other code;
  }

It's ok to use either bpf_cmp_likely or bpf_cmp_unlikely macros in both cases,
but consider that 'break' is effectively 'goto out_of_the_loop'.
Hence it's better to use bpf_cmp_unlikely in the B case.
While 'bar += foo' is better to keep as 'fallthrough' == likely code path in the A case.

When it's written as:
A.
  for() {
    if (bpf_cmp_likely(foo, >=, 10)) {
      bar += foo;
    }
    other code;
  }

B.
  for() {
    if (bpf_cmp_unlikely(foo, >=, 10))
       break;
    other code;
  }

The assembly will look like:
A.
  for() {
    if r1 < 10 goto L1;
      bar += foo;
  L1:
    other code;
  }

B.
  for() {
    if r1 >= 10 goto L2;
    other code;
  }
  L2:

The bpf_cmp_likely vs bpf_cmp_unlikely changes basic block layout, hence it will
greatly influence the verification process. The number of processed instructions
will be different, since the verifier walks the fallthrough first.

Signed-off-by: Alexei Starovoitov <[email protected]>
Signed-off-by: Andrii Nakryiko <[email protected]>
Acked-by: Daniel Borkmann <[email protected]>
Acked-by: Jiri Olsa <[email protected]>
Acked-by: Kumar Kartikeya Dwivedi <[email protected]>
Link: https://lore.kernel.org/bpf/[email protected]

Author: Alexei Starovoitov

tools/testing/selftests/bpf/bpf_experimental.h

@@ -254,6 +254,75 @@ extern void bpf_throw(u64 cookie) __ksym;
 		}									\
 	 })
 
+#define __cmp_cannot_be_signed(x) \
+	__builtin_strcmp(#x, "==") == 0 || __builtin_strcmp(#x, "!=") == 0 || \
+	__builtin_strcmp(#x, "&") == 0
+
+#define __is_signed_type(type) (((type)(-1)) < (type)1)
+
+#define __bpf_cmp(LHS, OP, SIGN, PRED, RHS, DEFAULT)						\
+	({											\
+		__label__ l_true;								\
+		bool ret = DEFAULT;								\
+		asm volatile goto("if %[lhs] " SIGN #OP " %[rhs] goto %l[l_true]"		\
+				  :: [lhs] "r"((short)LHS), [rhs] PRED (RHS) :: l_true);	\
+		ret = !DEFAULT;									\
+l_true:												\
+		ret;										\
+       })
+
+/* C type conversions coupled with comparison operator are tricky.
+ * Make sure BPF program is compiled with -Wsign-compare then
+ * __lhs OP __rhs below will catch the mistake.
+ * Be aware that we check only __lhs to figure out the sign of compare.
+ */
+#define _bpf_cmp(LHS, OP, RHS, NOFLIP)								\
+	({											\
+		typeof(LHS) __lhs = (LHS);							\
+		typeof(RHS) __rhs = (RHS);							\
+		bool ret;									\
+		_Static_assert(sizeof(&(LHS)), "1st argument must be an lvalue expression");	\
+		(void)(__lhs OP __rhs);								\
+		if (__cmp_cannot_be_signed(OP) || !__is_signed_type(typeof(__lhs))) {		\
+			if (sizeof(__rhs) == 8)							\
+				ret = __bpf_cmp(__lhs, OP, "", "r", __rhs, NOFLIP);		\
+			else									\
+				ret = __bpf_cmp(__lhs, OP, "", "i", __rhs, NOFLIP);		\
+		} else {									\
+			if (sizeof(__rhs) == 8)							\
+				ret = __bpf_cmp(__lhs, OP, "s", "r", __rhs, NOFLIP);		\
+			else									\
+				ret = __bpf_cmp(__lhs, OP, "s", "i", __rhs, NOFLIP);		\
+		}										\
+		ret;										\
+       })
+
+#ifndef bpf_cmp_unlikely
+#define bpf_cmp_unlikely(LHS, OP, RHS) _bpf_cmp(LHS, OP, RHS, true)
+#endif
+
+#ifndef bpf_cmp_likely
+#define bpf_cmp_likely(LHS, OP, RHS)								\
+	({											\
+		bool ret;									\
+		if (__builtin_strcmp(#OP, "==") == 0)						\
+			ret = _bpf_cmp(LHS, !=, RHS, false);					\
+		else if (__builtin_strcmp(#OP, "!=") == 0)					\
+			ret = _bpf_cmp(LHS, ==, RHS, false);					\
+		else if (__builtin_strcmp(#OP, "<=") == 0)					\
+			ret = _bpf_cmp(LHS, >, RHS, false);					\
+		else if (__builtin_strcmp(#OP, "<") == 0)					\
+			ret = _bpf_cmp(LHS, >=, RHS, false);					\
+		else if (__builtin_strcmp(#OP, ">") == 0)					\
+			ret = _bpf_cmp(LHS, <=, RHS, false);					\
+		else if (__builtin_strcmp(#OP, ">=") == 0)					\
+			ret = _bpf_cmp(LHS, <, RHS, false);					\
+		else										\
+			(void) "bug";								\
+		ret;										\
+       })
+#endif
+
 /* Description
  *	Assert that a conditional expression is true.
  * Returns

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

@@ -210,39 +210,39 @@ __noinline int assert_zero_gfunc(u64 c)
 {
 	volatile u64 cookie = c;
 
-	bpf_assert_eq(cookie, 0);
+	bpf_assert(bpf_cmp_unlikely(cookie, ==, 0));
 	return 0;
 }
 
 __noinline int assert_neg_gfunc(s64 c)
 {
 	volatile s64 cookie = c;
 
-	bpf_assert_lt(cookie, 0);
+	bpf_assert(bpf_cmp_unlikely(cookie, <, 0));
 	return 0;
 }
 
 __noinline int assert_pos_gfunc(s64 c)
 {
 	volatile s64 cookie = c;
 
-	bpf_assert_gt(cookie, 0);
+	bpf_assert(bpf_cmp_unlikely(cookie, >, 0));
 	return 0;
 }
 
 __noinline int assert_negeq_gfunc(s64 c)
 {
 	volatile s64 cookie = c;
 
-	bpf_assert_le(cookie, -1);
+	bpf_assert(bpf_cmp_unlikely(cookie, <=, -1));
 	return 0;
 }
 
 __noinline int assert_poseq_gfunc(s64 c)
 {
 	volatile s64 cookie = c;
 
-	bpf_assert_ge(cookie, 1);
+	bpf_assert(bpf_cmp_unlikely(cookie, >=, 1));
 	return 0;
 }
 
@@ -258,39 +258,39 @@ __noinline int assert_zero_gfunc_with(u64 c)
 {
 	volatile u64 cookie = c;
 
-	bpf_assert_eq_with(cookie, 0, cookie + 100);
+	bpf_assert_with(bpf_cmp_unlikely(cookie, ==, 0), cookie + 100);
 	return 0;
 }
 
 __noinline int assert_neg_gfunc_with(s64 c)
 {
 	volatile s64 cookie = c;
 
-	bpf_assert_lt_with(cookie, 0, cookie + 100);
+	bpf_assert_with(bpf_cmp_unlikely(cookie, <, 0), cookie + 100);
 	return 0;
 }
 
 __noinline int assert_pos_gfunc_with(s64 c)
 {
 	volatile s64 cookie = c;
 
-	bpf_assert_gt_with(cookie, 0, cookie + 100);
+	bpf_assert_with(bpf_cmp_unlikely(cookie, >, 0), cookie + 100);
 	return 0;
 }
 
 __noinline int assert_negeq_gfunc_with(s64 c)
 {
 	volatile s64 cookie = c;
 
-	bpf_assert_le_with(cookie, -1, cookie + 100);
+	bpf_assert_with(bpf_cmp_unlikely(cookie, <=, -1), cookie + 100);
 	return 0;
 }
 
 __noinline int assert_poseq_gfunc_with(s64 c)
 {
 	volatile s64 cookie = c;
 
-	bpf_assert_ge_with(cookie, 1, cookie + 100);
+	bpf_assert_with(bpf_cmp_unlikely(cookie, >=, 1), cookie + 100);
 	return 0;
 }
 

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

@@ -28,9 +28,8 @@ int iter_task_vma_for_each(const void *ctx)
 		return 0;
 
 	bpf_for_each(task_vma, vma, task, 0) {
-		if (seen >= 1000)
+		if (bpf_cmp_unlikely(seen, >=, 1000))
 			break;
-		barrier_var(seen);
 
 		vm_ranges[seen].vm_start = vma->vm_start;
 		vm_ranges[seen].vm_end = vma->vm_end;