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

["Emil Tsalapatis" <emil@etsalapatis.com>]

On Mon Dec 15, 2025 at 4:26 PM EST, Eduard Zingerman wrote:
> 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?
>

For sure, I will make sure the arena globals are writable.

>> +	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];
>                                             ^^^^^^^^^
>

Ack, will fix.

>> +
>> +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";