Re: [PATCH bpf-next] selftests/bpf: implement setting global variables in veristat

[Eduard Zingerman <eddyz87@gmail.com>]

On Mon, 2025-02-03 at 16:40 +0000, Mykyta Yatsenko wrote:
> From: Mykyta Yatsenko <yatsenko@meta.com>
> 
> To better verify some complex BPF programs we'd like to preset global
> variables.
> This patch introduces CLI argument `--set-global-vars` to veristat, that
> allows presetting values to global variables defined in BPF program. For
> example:
> 
> prog.c:
> ```
> enum Enum { ELEMENT1 = 0, ELEMENT2 = 5 };
> const volatile __s64 a = 5;
> const volatile __u8 b = 5;
> const volatile enum Enum c = ELEMENT2;
> const volatile bool d = false;
> 
> char arr[4] = {0};
> 
> SEC("tp_btf/sched_switch")
> int BPF_PROG(...)
> {
> 	bpf_printk("%c\n", arr[a]);
> 	bpf_printk("%c\n", arr[b]);
> 	bpf_printk("%c\n", arr[c]);
> 	bpf_printk("%c\n", arr[d]);
> 	return 0;
> }
> ```
> By default verification of the program fails:
> ```
> ./veristat prog.bpf.o
> ```
> By presetting global variables, we can make verification pass:
> ```
> ./veristat wq.bpf.o  --set-global-vars "a = 0; b = 1; c = 2; d = 3;"
> ```
> 
> Signed-off-by: Mykyta Yatsenko <yatsenko@meta.com>
> ---

This is super useful, thank you!
Maybe also add an ability to read variables list from a file?
(e.g. using -g @file-name syntax as in -f).

Worked fine for my small example, but failed to affect an object file
with multiple programs, see below.

Also, given that it is non-trivial to see if variable had indeed been set,
I think it would be useful to add a selftest that does
system("./veristat -l7 -v -g ...") and matches log output to check that
values are set correctly, e.g. I used the following simple test:

	const volatile u8  _u8  = 0;
	const volatile u16 _u16 = 0;
	const volatile u32 _u32 = 0;
	const volatile u64 _u64 = 0;
	const volatile s8  _s8  = 0;
	const volatile s16 _s16 = 0;
	const volatile s32 _s32 = 0;
	const volatile s64 _s64 = 0;

	SEC("socket")
	int test_globals(void *ctx)
	{
		volatile unsigned long cnt;
		cnt = _u8;
		cnt = _u16;
		cnt = _u32;
		cnt = _u64;
		cnt = _s8;
		cnt = _s16;
		cnt = _s32;
		cnt = _s64;
		return cnt;
	}

>  tools/testing/selftests/bpf/veristat.c | 189 +++++++++++++++++++++++++
>  1 file changed, 189 insertions(+)

[...]

> @@ -1292,6 +1312,169 @@ static int process_prog(const char *filename, struct bpf_object *obj, struct bpf
>  	return 0;
>  };
>  
> +static int parse_var_presets(char *expr, struct var_preset *presets, int capacity, int *size)
> +{
> +	char *state;
> +	char *next;
> +	int i = 0;
> +
> +	while ((next = strtok_r(i ? NULL : expr, ";", &state))) {
> +		char *eq_ptr = strchr(next, '=');
> +		char *name_ptr = next;
> +		char *name_end = eq_ptr - 1;
> +		char *val_ptr = eq_ptr + 1;
> +
> +		if (!eq_ptr)
> +			continue;

Nit: error message here?

> +
> +		if (i >= capacity) {
> +			fprintf(stderr, "Too many global variable presets\n");
> +			return -EINVAL;
> +		}
> +		while (isspace(*name_ptr))
> +			++name_ptr;
> +		while (isspace(*name_end))
> +			--name_end;
> +
> +		*(name_end + 1) = '\0';
> +		presets[i].name = strdup(name_ptr);
> +		errno = 0;
> +		presets[i].value = strtoll(val_ptr, NULL, 10);

Nit: using base of 0 would allow to specify values either as decimals or in hex
     (using '0x' prefix).

> +		if (errno == ERANGE) {
> +			errno = 0;
> +			presets[i].value = strtoull(val_ptr, NULL, 10);
> +		}
> +		if (errno) {
> +			fprintf(stderr, "Could not parse integer value %s\n", val_ptr);
> +			return -EINVAL;
> +		}
> +		++i;
> +	}
> +	*size = i;
> +	return 0;
> +}
> +
> +static bool is_signed_type(const struct btf_type *type)
> +{
> +	if (btf_is_int(type))

Nit: enums could be signed as well.

> +		return btf_int_encoding(type) & BTF_INT_SIGNED;
> +	return true;
> +}
> +
> +static const struct btf_type *var_base_type(const struct btf *btf, const struct btf_type *type)
> +{
> +	switch (btf_kind(type)) {
> +	case BTF_KIND_VAR:
> +	case BTF_KIND_TYPE_TAG:
> +	case BTF_KIND_CONST:
> +	case BTF_KIND_VOLATILE:
> +	case BTF_KIND_RESTRICT:
> +	case BTF_KIND_TYPEDEF:
> +	case BTF_KIND_DECL_TAG:
> +		return var_base_type(btf, btf__type_by_id(btf, type->type));
> +	}
> +	return type;
> +}
> +
> +static bool is_preset_supported(const struct btf_type *t)
> +{
> +	return btf_is_int(t) || btf_is_enum(t) || btf_is_enum64(t);
> +}
> +
> +static int set_global_var(struct bpf_object *obj, struct btf *btf, const struct btf_type *t,
> +			  struct bpf_map *map, struct btf_var_secinfo *sinfo, long long new_val)
> +{
> +	const struct btf_type *base_type;
> +	void *ptr;
> +	size_t size;
> +
> +	base_type = var_base_type(btf, t);
> +	if (!is_preset_supported(base_type)) {
> +		fprintf(stderr, "Setting global variable for btf kind %d is not supported\n",
> +			btf_kind(base_type));
> +		return -EINVAL;
> +	}
> +
> +	/* Check if value fits into the target variable size */
> +	if  (sinfo->size < sizeof(new_val)) {
> +		bool is_signed = is_signed_type(base_type);
> +		__u32 unsigned_bits = sinfo->size * 8 - (is_signed ? 1 : 0);
> +		long long max_val = 1ll << unsigned_bits;
> +
> +		if (new_val >= max_val || new_val < -max_val) {
> +			fprintf(stderr,
> +				"Variable %s value %lld is out of range [%lld; %lld]\n",
> +				btf__name_by_offset(btf, t->name_off), new_val,
> +				is_signed ? -max_val : 0, max_val - 1);
> +			return -EINVAL;
> +		}
> +	}
> +
> +	ptr = (void *)bpf_map__initial_value(map, &size);
> +	if (!ptr || (sinfo->offset + sinfo->size > size))
> +		return -EINVAL;
> +
> +	memcpy(ptr + sinfo->offset, &new_val, sinfo->size);

will this work for big endian?

> +	return 0;
> +}
> +
> +static int set_global_vars(struct bpf_object *obj, struct var_preset *presets, int npresets)
> +{
> +	struct btf_var_secinfo *sinfo;
> +	const char *sec_name;
> +	const struct btf_type *type;
> +	struct bpf_map *map;
> +	struct btf *btf;
> +	int i, j, k, n, cnt, err, preset_cnt = 0;
> +
> +	if (npresets == 0)
> +		return 0;
> +
> +	btf = bpf_object__btf(obj);
> +	if (!btf)
> +		return -EINVAL;
> +
> +	cnt = btf__type_cnt(btf);
> +	for (i  = 0; i != cnt; ++i) {
> +		type = btf__type_by_id(btf, i);
> +
> +		if (!btf_is_datasec(type))
> +			continue;
> +
> +		sinfo = btf_var_secinfos(type);
> +		sec_name = btf__name_by_offset(btf, type->name_off);
> +		map = bpf_object__find_map_by_name(obj, sec_name);
> +		if (!map)
> +			continue;
> +
> +		n = btf_vlen(type);
> +		for (j = 0; j < n; ++j, ++sinfo) {
> +			const struct btf_type *var_type = btf__type_by_id(btf, sinfo->type);
> +			const char *var_name = btf__name_by_offset(btf, var_type->name_off);
> +
> +			if (!btf_is_var(var_type))
> +				continue;
> +
> +			for (k = 0; k < npresets; ++k) {
> +				if (strcmp(var_name, presets[k].name) != 0)
> +					continue;
> +
> +				err = set_global_var(obj, btf, var_type, map, sinfo,
> +						     presets[k].value);
> +				if (err)
> +					return err;
> +
> +				preset_cnt++;
> +				break;
> +			}
> +		}
> +	}
> +	if (preset_cnt != npresets)
> +		fprintf(stderr, "Some global variable presets have not been applied\n");

Nit: it would be nice to print which ones were not set.

> +
> +	return 0;
> +}
> +
>  static int process_obj(const char *filename)
>  {
>  	const char *base_filename = basename(strdupa(filename));
> @@ -1338,6 +1521,12 @@ static int process_obj(const char *filename)
>  		prog_cnt++;
>  	}
>  
> +	err = set_global_vars(obj, env.presets, env.npresets);
> +	if (err) {
> +		fprintf(stderr, "Failed to set global variables\n");
> +		goto cleanup;
> +	}
> +
>  	if (prog_cnt == 1) {
>  		prog = bpf_object__next_program(obj, NULL);
>  		bpf_program__set_autoload(prog, true);

Same needs to happen for the loop below when prog_cnt != 1, e.g.:

@@ -1544,6 +1544,12 @@ static int process_obj(const char *filename)
                        goto cleanup;
                }
 
+               err = set_global_vars(tobj, env.presets, env.npresets);
+               if (err) {
+                       fprintf(stderr, "Failed to set global variables\n");
+                       goto cleanup;
+               }
+
                lprog = NULL;
                bpf_object__for_each_program(tprog, tobj) {
                        const char *tprog_name = bpf_program__name(tprog);

Or, better yet, get rid of the `prog_cnt == 1` special case.