[PATCH v2 1/4] sched_ext: Don't sign-extend unsigned enums in __COMPAT_read_enum()

[Tejun Heo <tj@kernel.org>]

__COMPAT_read_enum() looks an enumerator up in the running kernel's BTF. For
a 4-byte enum, it assigns struct btf_enum.val, an __s32, to the u64 result,
which sign-extends. BTF records the enum's signedness in the type's kflag.
The code ignores it, so an unsigned enumerator with bit 31 set is
sign-extended too.

b1c2a6b0d90d ("sched_ext: scx_qmap: Read SCX_RESCUE_DISABLE through the enum
lookup tables") added SCX_RESCUE_DISABLE to the enum lookup tables. It is
U32_MAX in enum scx_consts and the only such enumerator in the tables. BPF
code sees it as 0xffffffffffffffff instead of 0xffffffff, so a comparison
against a u32 holding the real value never matches.

scx_qmap, its only user, narrows the value to u32 in userspace and is
unaffected.

Sign-extend only when the kflag says the enum is signed and zero-extend
otherwise. A signed enum keeps the sign extension, which is what C gives
when the enumerator converts to u64. SCX_TASK_CURSOR, 1 << 31 in a signed
enum, still reads 0xffffffff80000000. Verified with a probe against a built
kernel's BTF.

v2: Note in the comment that pahole < 1.24 never sets the kflag (Andrea).

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Andrea Righi <arighi@nvidia.com>
---
Applied to sched_ext/for-7.5.

 tools/sched_ext/include/scx/compat.h | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/tools/sched_ext/include/scx/compat.h b/tools/sched_ext/include/scx/compat.h
index 07fb55e63c57..8a17854cc626 100644
--- a/tools/sched_ext/include/scx/compat.h
+++ b/tools/sched_ext/include/scx/compat.h
@@ -143,7 +143,15 @@ static inline bool __COMPAT_read_enum(const char *type, const char *name, u64 *v
 				if (t->size == 8 && !strncmp(type, "scx_", 4))
 					return __COMPAT_recover_truncated_enum64(type, name,
 										 (u32)e[i].val, v);
-				*v = e[i].val;
+				/*
+				 * kflag marks a signed enum. BTF generated by
+				 * pahole < 1.24 never sets it, so signed enums
+				 * are zero-extended there too.
+				 */
+				if (btf_kflag(t))
+					*v = (s64)e[i].val;
+				else
+					*v = (u32)e[i].val;
 				return true;
 			}
 		}
-- 
2.56.0