selftests/bpf: add fp-leaking precise subprog result tests

Add selftests validating that BPF verifier handles precision marking
for SCALAR registers derived from r10 (fp) register correctly.

Given `r0 = (s8)r10;` syntax is not supported by older Clang compilers,
use the raw BPF instruction syntax to maximize compatibility.
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20240404214536.3551295-2-andrii@kernel.orgSigned-off-by: Alexei Starovoitov <ast@kernel.org>

tools/include/linux/filter.h

@@ -111,6 +111,24 @@
 		.off   = 0,					\
 		.imm   = IMM })
 
+/* Short form of movsx, dst_reg = (s8,s16,s32)src_reg */
+
+#define BPF_MOVSX64_REG(DST, SRC, OFF)				\
+	((struct bpf_insn) {					\
+		.code  = BPF_ALU64 | BPF_MOV | BPF_X,		\
+		.dst_reg = DST,					\
+		.src_reg = SRC,					\
+		.off   = OFF,					\
+		.imm   = 0 })
+
+#define BPF_MOVSX32_REG(DST, SRC, OFF)				\
+	((struct bpf_insn) {					\
+		.code  = BPF_ALU | BPF_MOV | BPF_X,		\
+		.dst_reg = DST,					\
+		.src_reg = SRC,					\
+		.off   = OFF,					\
+		.imm   = 0 })
+
 /* Short form of mov based on type,  BPF_X: dst_reg = src_reg, BPF_K: dst_reg = imm32 */
 
 #define BPF_MOV64_RAW(TYPE, DST, SRC, IMM)			\

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

@@ -6,6 +6,7 @@
 #include <linux/bpf.h>
 #include <bpf/bpf_helpers.h>
 #include "bpf_misc.h"
+#include <../../../tools/include/linux/filter.h>
 
 #define ARRAY_SIZE(x) (sizeof(x) / sizeof(x[0]))
 
@@ -76,6 +77,94 @@ __naked int subprog_result_precise(void)
 	);
 }
 
+__naked __noinline __used
+static unsigned long fp_leaking_subprog()
+{
+	asm volatile (
+		".8byte %[r0_eq_r10_cast_s8];"
+		"exit;"
+		:: __imm_insn(r0_eq_r10_cast_s8, BPF_MOVSX64_REG(BPF_REG_0, BPF_REG_10, 8))
+	);
+}
+
+__naked __noinline __used
+static unsigned long sneaky_fp_leaking_subprog()
+{
+	asm volatile (
+		"r1 = r10;"
+		".8byte %[r0_eq_r1_cast_s8];"
+		"exit;"
+		:: __imm_insn(r0_eq_r1_cast_s8, BPF_MOVSX64_REG(BPF_REG_0, BPF_REG_1, 8))
+	);
+}
+
+SEC("?raw_tp")
+__success __log_level(2)
+__msg("6: (0f) r1 += r0")
+__msg("mark_precise: frame0: last_idx 6 first_idx 0 subseq_idx -1")
+__msg("mark_precise: frame0: regs=r0 stack= before 5: (bf) r1 = r6")
+__msg("mark_precise: frame0: regs=r0 stack= before 4: (27) r0 *= 4")
+__msg("mark_precise: frame0: regs=r0 stack= before 3: (57) r0 &= 3")
+__msg("mark_precise: frame0: regs=r0 stack= before 10: (95) exit")
+__msg("mark_precise: frame1: regs=r0 stack= before 9: (bf) r0 = (s8)r10")
+__msg("7: R0_w=scalar")
+__naked int fp_precise_subprog_result(void)
+{
+	asm volatile (
+		"call fp_leaking_subprog;"
+		/* use subprog's returned value (which is derived from r10=fp
+		 * register), as index into vals array, forcing all of that to
+		 * be known precisely
+		 */
+		"r0 &= 3;"
+		"r0 *= 4;"
+		"r1 = %[vals];"
+		/* force precision marking */
+		"r1 += r0;"
+		"r0 = *(u32 *)(r1 + 0);"
+		"exit;"
+		:
+		: __imm_ptr(vals)
+		: __clobber_common
+	);
+}
+
+SEC("?raw_tp")
+__success __log_level(2)
+__msg("6: (0f) r1 += r0")
+__msg("mark_precise: frame0: last_idx 6 first_idx 0 subseq_idx -1")
+__msg("mark_precise: frame0: regs=r0 stack= before 5: (bf) r1 = r6")
+__msg("mark_precise: frame0: regs=r0 stack= before 4: (27) r0 *= 4")
+__msg("mark_precise: frame0: regs=r0 stack= before 3: (57) r0 &= 3")
+__msg("mark_precise: frame0: regs=r0 stack= before 11: (95) exit")
+__msg("mark_precise: frame1: regs=r0 stack= before 10: (bf) r0 = (s8)r1")
+/* here r1 is marked precise, even though it's fp register, but that's fine
+ * because by the time we get out of subprogram it has to be derived from r10
+ * anyways, at which point we'll break precision chain
+ */
+__msg("mark_precise: frame1: regs=r1 stack= before 9: (bf) r1 = r10")
+__msg("7: R0_w=scalar")
+__naked int sneaky_fp_precise_subprog_result(void)
+{
+	asm volatile (
+		"call sneaky_fp_leaking_subprog;"
+		/* use subprog's returned value (which is derived from r10=fp
+		 * register), as index into vals array, forcing all of that to
+		 * be known precisely
+		 */
+		"r0 &= 3;"
+		"r0 *= 4;"
+		"r1 = %[vals];"
+		/* force precision marking */
+		"r1 += r0;"
+		"r0 = *(u32 *)(r1 + 0);"
+		"exit;"
+		:
+		: __imm_ptr(vals)
+		: __clobber_common
+	);
+}
+
 SEC("?raw_tp")
 __success __log_level(2)
 __msg("9: (0f) r1 += r0")