Re: [PATCH v3 5/5] selftests/bpf: add tests for the arena offset of globals

[Eduard Zingerman <eddyz87@gmail.com>]

On Mon, 2025-12-15 at 11:13 -0500, Emil Tsalapatis wrote:
> Add tests for the new libbpf globals arena offset logic. The
> tests cover the case of globals being as large as the arena
> itself, and being smaller than the arena. In that case, the
> data is placed at the end of the arena, and the beginning
> of the arena is free.
> 
> Signed-off-by: Emil Tsalapatis <emil@etsalapatis.com>
> ---
>  .../selftests/bpf/prog_tests/verifier.c       |  4 +
>  .../bpf/progs/verifier_arena_globals1.c       | 75 +++++++++++++++++++
>  .../bpf/progs/verifier_arena_globals2.c       | 49 ++++++++++++
>  3 files changed, 128 insertions(+)
>  create mode 100644 tools/testing/selftests/bpf/progs/verifier_arena_globals1.c
>  create mode 100644 tools/testing/selftests/bpf/progs/verifier_arena_globals2.c
> 
> diff --git a/tools/testing/selftests/bpf/prog_tests/verifier.c b/tools/testing/selftests/bpf/prog_tests/verifier.c
> index 4b4b081b46cc..5829ffd70f8f 100644
> --- a/tools/testing/selftests/bpf/prog_tests/verifier.c
> +++ b/tools/testing/selftests/bpf/prog_tests/verifier.c
> @@ -6,6 +6,8 @@
>  #include "verifier_and.skel.h"
>  #include "verifier_arena.skel.h"
>  #include "verifier_arena_large.skel.h"
> +#include "verifier_arena_globals1.skel.h"
> +#include "verifier_arena_globals2.skel.h"
>  #include "verifier_array_access.skel.h"
>  #include "verifier_async_cb_context.skel.h"
>  #include "verifier_basic_stack.skel.h"
> @@ -147,6 +149,8 @@ static void run_tests_aux(const char *skel_name,
>  void test_verifier_and(void)                  { RUN(verifier_and); }
>  void test_verifier_arena(void)                { RUN(verifier_arena); }
>  void test_verifier_arena_large(void)          { RUN(verifier_arena_large); }
> +void test_verifier_arena_globals1(void)       { RUN(verifier_arena_globals1); }
> +void test_verifier_arena_globals2(void)       { RUN(verifier_arena_globals2); }
>  void test_verifier_basic_stack(void)          { RUN(verifier_basic_stack); }
>  void test_verifier_bitfield_write(void)       { RUN(verifier_bitfield_write); }
>  void test_verifier_bounds(void)               { RUN(verifier_bounds); }
> diff --git a/tools/testing/selftests/bpf/progs/verifier_arena_globals1.c b/tools/testing/selftests/bpf/progs/verifier_arena_globals1.c
> new file mode 100644
> index 000000000000..d998a277e5e7
> --- /dev/null
> +++ b/tools/testing/selftests/bpf/progs/verifier_arena_globals1.c
> @@ -0,0 +1,75 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/* Copyright (c) 2025 Meta Platforms, Inc. and affiliates. */
> +
> +#define BPF_NO_KFUNC_PROTOTYPES
> +#include <vmlinux.h>
> +#include <bpf/bpf_helpers.h>
> +#include <bpf/bpf_tracing.h>
> +#include "bpf_experimental.h"
> +#include "bpf_arena_common.h"
> +#include "bpf_misc.h"
> +
> +#define ARENA_PAGES (1UL<< (32 - 12))
> +#define GLOBAL_PAGES (16)
> +
> +struct {
> +	__uint(type, BPF_MAP_TYPE_ARENA);
> +	__uint(map_flags, BPF_F_MMAPABLE);
> +	__uint(max_entries, ARENA_PAGES);
> +#ifdef __TARGET_ARCH_arm64
> +	__ulong(map_extra, (1ull << 32) | (~0u - __PAGE_SIZE * ARENA_PAGES + 1));
> +#else
> +	__ulong(map_extra, (1ull << 44) | (~0u - __PAGE_SIZE * ARENA_PAGES + 1));
> +#endif
> +} arena SEC(".maps");
> +
> +/*
> + * Global data, to be placed at the end of the arena.
> + */
> +char __arena global_data[GLOBAL_PAGES][PAGE_SIZE];
> +
> +SEC("syscall")
> +__success __retval(0)
> +int check_reserve1(void *ctx)
> +{
> +#if defined(__BPF_FEATURE_ADDR_SPACE_CAST)
> +	__u8 __arena *guard, *globals;
> +	int ret;
> +
> +	guard = (void __arena *)arena_base(&arena);
> +	globals = (void __arena *)(arena_base(&arena) + (ARENA_PAGES - GLOBAL_PAGES) * PAGE_SIZE);
> +
> +	/* Reserve the region we've offset the globals by. */
> +	ret = bpf_arena_reserve_pages(&arena, guard, ARENA_PAGES - GLOBAL_PAGES);
> +	if (ret)
> +		return 1;
> +
> +	/* Make sure the globals are in the expected offset. */
> +	ret = bpf_arena_reserve_pages(&arena, globals, 1);

In addition to checking that reserving pages succeeds,
do we need to test pages content here?

> +	if (!ret)
> +		return 2;
> +#endif
> +	return 0;
> +}
> +
> +/*
> + * Relocation check by reading directly into the global data w/o using symbols.
> + */
> +SEC("syscall")
> +__success __retval(0)
> +int check_relocation(void *ctx)
> +{
> +#if defined(__BPF_FEATURE_ADDR_SPACE_CAST)
> +	const u8 magic = 0xfa;
> +	u8 __arena *ptr;
> +
> +	global_data[GLOBAL_PAGES - 1][PAGE_SIZE / 2] = magic;
> +	ptr = (u8 __arena *)((u64)(ARENA_PAGES * PAGE_SIZE - PAGE_SIZE / 2));
> +	if (*ptr != magic)
> +		return 1;
> +
> +#endif
> +	return 0;
> +}
> +
> +char _license[] SEC("license") = "GPL";
> diff --git a/tools/testing/selftests/bpf/progs/verifier_arena_globals2.c b/tools/testing/selftests/bpf/progs/verifier_arena_globals2.c
> new file mode 100644
> index 000000000000..5a6f6bc3b00c
> --- /dev/null
> +++ b/tools/testing/selftests/bpf/progs/verifier_arena_globals2.c
> @@ -0,0 +1,49 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/* Copyright (c) 2025 Meta Platforms, Inc. and affiliates. */
> +
> +#define BPF_NO_KFUNC_PROTOTYPES
> +#include <vmlinux.h>
> +#include <bpf/bpf_helpers.h>
> +#include <bpf/bpf_tracing.h>
> +#include "bpf_misc.h"
> +#include "bpf_experimental.h"
> +#include "bpf_arena_common.h"
> +
> +#define ARENA_PAGES (32)
> +
> +struct {
> +	__uint(type, BPF_MAP_TYPE_ARENA);
> +	__uint(map_flags, BPF_F_MMAPABLE);
> +	__uint(max_entries, ARENA_PAGES);
> +#ifdef __TARGET_ARCH_arm64
> +	__ulong(map_extra, (1ull << 32) | (~0u - __PAGE_SIZE * ARENA_PAGES + 1));
> +#else
> +	__ulong(map_extra, (1ull << 44) | (~0u - __PAGE_SIZE * ARENA_PAGES + 1));
> +#endif
> +} arena SEC(".maps");
> +
> +/*
> + * Fill the entire arena with global data.
> + * The offset into the arena should be 0.
> + */
> +char __arena global_data[PAGE_SIZE][ARENA_PAGES];
                            ^^^^^^^^^

Nit: this is reversed compared to the declaration in the previous test:

  > +/*
  > + * Global data, to be placed at the end of the arena.
  > + */
  > +char __arena global_data[GLOBAL_PAGES][PAGE_SIZE];
                                            ^^^^^^^^^

> +
> +SEC("syscall")
> +__success __retval(0)
> +int check_reserve2(void *ctx)
> +{
> +#if defined(__BPF_FEATURE_ADDR_SPACE_CAST)
> +	void __arena *guard;
> +	int ret;
> +
> +	guard = (void __arena *)arena_base(&arena);
> +
> +	/* Make sure the data at offset 0 case is properly handled. */
> +	ret = bpf_arena_reserve_pages(&arena, guard, 1);
> +	if (!ret)
> +		return 1;
> +#endif
> +	return 0;
> +}
> +
> +char _license[] SEC("license") = "GPL";