bitfield.h: don't compile-time validate _val in FIELD_FIT

When ur_load_imm_any() is inlined into jeq_imm(), it's possible for the
compiler to deduce a case where _val can only have the value of -1 at
compile time. Specifically,

/* struct bpf_insn: _s32 imm */
u64 imm = insn->imm; /* sign extend */
if (imm >> 32) { /* non-zero only if insn->imm is negative */
  /* inlined from ur_load_imm_any */
  u32 __imm = imm >> 32; /* therefore, always 0xffffffff */
  if (__builtin_constant_p(__imm) && __imm > 255)
    compiletime_assert_XXX()

This can result in tripping a BUILD_BUG_ON() in __BF_FIELD_CHECK() that
checks that a given value is representable in one byte (interpreted as
unsigned).

FIELD_FIT() should return true or false at runtime for whether a value
can fit for not. Don't break the build over a value that's too large for
the mask. We'd prefer to keep the inlining and compiler optimizations
though we know this case will always return false.

Cc: [email protected]
Fixes: 1697599 ("bitfield.h: add FIELD_FIT() helper")
Link: https://lore.kernel.org/kernel-hardening/CAK7LNASvb0UDJ0U5wkYYRzTAdnEs64HjXpEUL7d=V0CXiAXcNw@mail.gmail.com/
Reported-by: Masahiro Yamada <[email protected]>
Debugged-by: Sami Tolvanen <[email protected]>
Signed-off-by: Jakub Kicinski <[email protected]>
Signed-off-by: Nick Desaulniers <[email protected]>
Signed-off-by: David S. Miller <[email protected]>

Author: kuba-moo

include/linux/bitfield.h

@@ -77,7 +77,7 @@
  */
 #define FIELD_FIT(_mask, _val)						\
 	({								\
-		__BF_FIELD_CHECK(_mask, 0ULL, _val, "FIELD_FIT: ");	\
+		__BF_FIELD_CHECK(_mask, 0ULL, 0ULL, "FIELD_FIT: ");	\
 		!((((typeof(_mask))_val) << __bf_shf(_mask)) & ~(_mask)); \
 	})