Merge branch 'compare-stack-frames-in-exact-register-states'

Kumar Kartikeya Dwivedi says:

====================
Compare stack frames in exact register states

regs_exact() compares register values and their ID relationships, but it
does not compare frameno. regsafe() checks frameno for ordinary
PTR_TO_STACK comparisons, while its EXACT path returns through regs_exact()
before reaching that check. Infinite-loop detection can therefore mistake
pointers to the same offset in different stack frames for the same pointer
and reject a finite loop.

Move frameno into bpf_reg_state's type-specific metadata union so the
existing regs_exact() prefix comparison covers it. This avoids a separate
PTR_TO_STACK case and keeps the structure at 80 bytes. Adjust the
states_maybe_looping() comparison boundary for the new layout. Since
frameno now aliases other pointer metadata, bpf_func() returns NULL for
registers that are not stack pointers; the callers that look up the frame
before checking the register type dereference it only afterwards.

The selftest keeps a stack pointer live in a register across a loop
whose only change at the header is the pointer's frame number. On the
unfixed tree, the program is rejected with "infinite loop detected". With
the fix, it loads and returns the expected value.

Changelog:
----------
v3 -> v4
v3: https://lore.kernel.org/bpf/20260919004327.1403382-1-memxor@gmail.com

 * Return NULL from bpf_func() for non-stack registers, since frameno now
   aliases other pointer metadata and some callers look up the frame before
   checking the register type. (Sashiko)

v2 -> v3
v2: https://lore.kernel.org/bpf/20260918011313.3053497-1-memxor@gmail.com

 * Rebase on bpf/master.
 * Drop the redundant spilled-pointer test, since existing tests already
   cover the stacksafe() -> regsafe() path. (Eduard)
 * Place asm labels on their own line in the selftest. (Eduard)
 * Collect Acked-by and Tested-by tags.

v1 -> v2
v1: https://lore.kernel.org/bpf/20260914161340.3419141-1-memxor@gmail.com

 * Rebase on bpf/master.
 * Move frameno into the type-specific metadata union so regs_exact()'s
   existing prefix comparison covers it without growing bpf_reg_state.
====================

Link: https://patch.msgid.link/20260919014213.1840880-1-memxor@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

1 files changed, 36 insertions, 0 deletions

diff --git a/tools/testing/selftests/bpf/progs/verifier_loops1.c b/tools/testing/selftests/bpf/progs/verifier_loops1.c
index d248ce877f14..48a966cda199 100644
--- a/tools/testing/selftests/bpf/progs/verifier_loops1.c
+++ b/tools/testing/selftests/bpf/progs/verifier_loops1.c
@@ -303,4 +303,40 @@ __naked void maybe_exit_scc_bug1(void)
 	::: __clobber_all);
 }
 
+/*
+ * The loop reads zero from the caller's stack on its first iteration and
+ * one from the callee's stack on its second iteration. At the loop header,
+ * only the frame number of the pointer in r1 changes.
+ */
+static __naked __noinline __used
+void loop_stack_frames_reg(void)
+{
+	asm volatile (
+	"*(u64 *)(r10 - 8) = 1;"
+"1:"
+	"r0 = *(u64 *)(r1 + 0);"
+	"if r0 != 0 goto 2f;"
+	"r1 = r10;"
+	"r1 += -8;"
+	"goto 1b;"
+"2:"
+	"exit;"
+	::: __clobber_all);
+}
+
+SEC("xdp")
+__description("bounded loop changing stack frame in a register")
+__success __retval(1)
+__flag(BPF_F_TEST_STATE_FREQ)
+__naked void bounded_loop_stack_frames_reg(void)
+{
+	asm volatile (
+	"*(u64 *)(r10 - 8) = 0;"
+	"r1 = r10;"
+	"r1 += -8;"
+	"call loop_stack_frames_reg;"
+	"exit;"
+	::: __clobber_all);
+}
+
 char _license[] SEC("license") = "GPL";