[PATCH v2 bpf-next 0/2] Strip away modifiers from BPF skeleton global variables

[PATCH v2 bpf-next 0/2] Strip away modifiers from BPF skeleton global variables

[Andrii Nakryiko]

Fix bpftool logic of stripping away const/volatile modifiers for all global
variables during BPF skeleton generation. See patch #1 for details on when
existing logic breaks and why it's important. Support special .strip_mods=true
mode in btf_dump__emit_type_decl.

Recent example of when this has caused problems can be found in [0].

  [0] https://github.com/iovisor/bcc/pull/2994#issuecomment-650588533

Cc: Anton Protopopov <a.s.protopopov@gmail.com>

Andrii Nakryiko (2):
  libbpf: support stripping modifiers for btf_dump
  tools/bpftool: strip away modifiers from global variables

 tools/bpf/bpftool/gen.c                       | 23 ++++++++-----------
 tools/lib/bpf/btf.h                           |  4 +++-
 tools/lib/bpf/btf_dump.c                      | 10 ++++++--
 .../selftests/bpf/prog_tests/skeleton.c       |  6 ++---
 .../selftests/bpf/progs/test_skeleton.c       |  6 +++--
 5 files changed, 28 insertions(+), 21 deletions(-)

-- 
2.24.1

[PATCH v2 bpf-next 1/2] libbpf: support stripping modifiers for btf_dump

[Andrii Nakryiko]

One important use case when emitting const/volatile/restrict is undesirable is
BPF skeleton generation of DATASEC layout. These are further memory-mapped and
can be written/read from user-space directly.

For important case of .rodata variables, bpftool strips away first-level
modifiers, to make their use on user-space side simple and not requiring extra
type casts to override compiler complaining about writing to const variables.

This logic works mostly fine, but breaks in some more complicated cases. E.g.:

    const volatile int params[10];

Because in BTF it's a chain of ARRAY -> CONST -> VOLATILE -> INT, bpftool
stops at ARRAY and doesn't strip CONST and VOLATILE. In skeleton this variable
will be emitted as is. So when used from user-space, compiler will complain
about writing to const array. This is problematic, as also mentioned in [0].

To solve this for arrays and other non-trivial cases (e.g., inner
const/volatile fields inside the struct), teach btf_dump to strip away any
modifier, when requested. This is done as an extra option on
btf_dump__emit_type_decl() API.

Reported-by: Anton Protopopov <a.s.protopopov@gmail.com>
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
---
 tools/lib/bpf/btf.h      |  2 ++
 tools/lib/bpf/btf_dump.c | 10 ++++++++--
 2 files changed, 10 insertions(+), 2 deletions(-)

diff --git a/tools/lib/bpf/btf.h b/tools/lib/bpf/btf.h
index 70c1b7ec2bd0..be98dd75b791 100644
--- a/tools/lib/bpf/btf.h
+++ b/tools/lib/bpf/btf.h
@@ -143,6 +143,8 @@ struct btf_dump_emit_type_decl_opts {
 	 * necessary indentation already
 	 */
 	int indent_level;
+	/* strip all the const/volatile/restrict mods */
+	bool strip_mods;
 };
 #define btf_dump_emit_type_decl_opts__last_field indent_level
 
diff --git a/tools/lib/bpf/btf_dump.c b/tools/lib/bpf/btf_dump.c
index bbb430317260..e1c344504cae 100644
--- a/tools/lib/bpf/btf_dump.c
+++ b/tools/lib/bpf/btf_dump.c
@@ -60,6 +60,7 @@ struct btf_dump {
 	const struct btf_ext *btf_ext;
 	btf_dump_printf_fn_t printf_fn;
 	struct btf_dump_opts opts;
+	bool strip_mods;
 
 	/* per-type auxiliary state */
 	struct btf_dump_type_aux_state *type_states;
@@ -1032,7 +1033,9 @@ int btf_dump__emit_type_decl(struct btf_dump *d, __u32 id,
 
 	fname = OPTS_GET(opts, field_name, "");
 	lvl = OPTS_GET(opts, indent_level, 0);
+	d->strip_mods = OPTS_GET(opts, strip_mods, false);
 	btf_dump_emit_type_decl(d, id, fname, lvl);
+	d->strip_mods = false;
 	return 0;
 }
 
@@ -1045,6 +1048,10 @@ static void btf_dump_emit_type_decl(struct btf_dump *d, __u32 id,
 
 	stack_start = d->decl_stack_cnt;
 	for (;;) {
+		t = btf__type_by_id(d->btf, id);
+		if (d->strip_mods && btf_is_mod(t))
+			goto skip_mod;
+
 		err = btf_dump_push_decl_stack_id(d, id);
 		if (err < 0) {
 			/*
@@ -1056,12 +1063,11 @@ static void btf_dump_emit_type_decl(struct btf_dump *d, __u32 id,
 			d->decl_stack_cnt = stack_start;
 			return;
 		}
-
+skip_mod:
 		/* VOID */
 		if (id == 0)
 			break;
 
-		t = btf__type_by_id(d->btf, id);
 		switch (btf_kind(t)) {
 		case BTF_KIND_PTR:
 		case BTF_KIND_VOLATILE:
-- 
2.24.1

[PATCH v2 bpf-next 2/2] tools/bpftool: strip away modifiers from global variables

[Andrii Nakryiko]

Reliably remove all the type modifiers from read-only (.rodata) global
variable definitions, including cases of inner field const modifiers and
arrays of const values.

Also modify one of selftests to ensure that const volatile struct doesn't
prevent user-space from modifying .rodata variable.

Fixes: 985ead416df3 ("bpftool: Add skeleton codegen command")
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
---
 tools/bpf/bpftool/gen.c                       | 23 ++++++++-----------
 tools/lib/bpf/btf.h                           |  2 +-
 .../selftests/bpf/prog_tests/skeleton.c       |  6 ++---
 .../selftests/bpf/progs/test_skeleton.c       |  6 +++--
 4 files changed, 18 insertions(+), 19 deletions(-)

diff --git a/tools/bpf/bpftool/gen.c b/tools/bpf/bpftool/gen.c
index 10de76b296ba..b59d26e89367 100644
--- a/tools/bpf/bpftool/gen.c
+++ b/tools/bpf/bpftool/gen.c
@@ -88,7 +88,7 @@ static const char *get_map_ident(const struct bpf_map *map)
 		return NULL;
 }
 
-static void codegen_btf_dump_printf(void *ct, const char *fmt, va_list args)
+static void codegen_btf_dump_printf(void *ctx, const char *fmt, va_list args)
 {
 	vprintf(fmt, args);
 }
@@ -104,17 +104,20 @@ static int codegen_datasec_def(struct bpf_object *obj,
 	int i, err, off = 0, pad_cnt = 0, vlen = btf_vlen(sec);
 	const char *sec_ident;
 	char var_ident[256];
+	bool strip_mods = false;
 
-	if (strcmp(sec_name, ".data") == 0)
+	if (strcmp(sec_name, ".data") == 0) {
 		sec_ident = "data";
-	else if (strcmp(sec_name, ".bss") == 0)
+	} else if (strcmp(sec_name, ".bss") == 0) {
 		sec_ident = "bss";
-	else if (strcmp(sec_name, ".rodata") == 0)
+	} else if (strcmp(sec_name, ".rodata") == 0) {
 		sec_ident = "rodata";
-	else if (strcmp(sec_name, ".kconfig") == 0)
+		strip_mods = true;
+	} else if (strcmp(sec_name, ".kconfig") == 0) {
 		sec_ident = "kconfig";
-	else
+	} else {
 		return 0;
+	}
 
 	printf("	struct %s__%s {\n", obj_name, sec_ident);
 	for (i = 0; i < vlen; i++, sec_var++) {
@@ -123,16 +126,10 @@ static int codegen_datasec_def(struct bpf_object *obj,
 		DECLARE_LIBBPF_OPTS(btf_dump_emit_type_decl_opts, opts,
 			.field_name = var_ident,
 			.indent_level = 2,
+			.strip_mods = strip_mods,
 		);
 		int need_off = sec_var->offset, align_off, align;
 		__u32 var_type_id = var->type;
-		const struct btf_type *t;
-
-		t = btf__type_by_id(btf, var_type_id);
-		while (btf_is_mod(t)) {
-			var_type_id = t->type;
-			t = btf__type_by_id(btf, var_type_id);
-		}
 
 		if (off > need_off) {
 			p_err("Something is wrong for %s's variable #%d: need offset %d, already at %d.\n",
diff --git a/tools/lib/bpf/btf.h b/tools/lib/bpf/btf.h
index be98dd75b791..7f003bbeb35e 100644
--- a/tools/lib/bpf/btf.h
+++ b/tools/lib/bpf/btf.h
@@ -146,7 +146,7 @@ struct btf_dump_emit_type_decl_opts {
 	/* strip all the const/volatile/restrict mods */
 	bool strip_mods;
 };
-#define btf_dump_emit_type_decl_opts__last_field indent_level
+#define btf_dump_emit_type_decl_opts__last_field strip_mods
 
 LIBBPF_API int
 btf_dump__emit_type_decl(struct btf_dump *d, __u32 id,
diff --git a/tools/testing/selftests/bpf/prog_tests/skeleton.c b/tools/testing/selftests/bpf/prog_tests/skeleton.c
index fa153cf67b1b..fe87b77af459 100644
--- a/tools/testing/selftests/bpf/prog_tests/skeleton.c
+++ b/tools/testing/selftests/bpf/prog_tests/skeleton.c
@@ -41,7 +41,7 @@ void test_skeleton(void)
 	CHECK(bss->in4 != 0, "in4", "got %lld != exp %lld\n", bss->in4, 0LL);
 	CHECK(bss->out4 != 0, "out4", "got %lld != exp %lld\n", bss->out4, 0LL);
 
-	CHECK(rodata->in6 != 0, "in6", "got %d != exp %d\n", rodata->in6, 0);
+	CHECK(rodata->in.in6 != 0, "in6", "got %d != exp %d\n", rodata->in.in6, 0);
 	CHECK(bss->out6 != 0, "out6", "got %d != exp %d\n", bss->out6, 0);
 
 	/* validate we can pre-setup global variables, even in .bss */
@@ -49,7 +49,7 @@ void test_skeleton(void)
 	data->in2 = 11;
 	bss->in3 = 12;
 	bss->in4 = 13;
-	rodata->in6 = 14;
+	rodata->in.in6 = 14;
 
 	err = test_skeleton__load(skel);
 	if (CHECK(err, "skel_load", "failed to load skeleton: %d\n", err))
@@ -60,7 +60,7 @@ void test_skeleton(void)
 	CHECK(data->in2 != 11, "in2", "got %lld != exp %lld\n", data->in2, 11LL);
 	CHECK(bss->in3 != 12, "in3", "got %d != exp %d\n", bss->in3, 12);
 	CHECK(bss->in4 != 13, "in4", "got %lld != exp %lld\n", bss->in4, 13LL);
-	CHECK(rodata->in6 != 14, "in6", "got %d != exp %d\n", rodata->in6, 14);
+	CHECK(rodata->in.in6 != 14, "in6", "got %d != exp %d\n", rodata->in.in6, 14);
 
 	/* now set new values and attach to get them into outX variables */
 	data->in1 = 1;
diff --git a/tools/testing/selftests/bpf/progs/test_skeleton.c b/tools/testing/selftests/bpf/progs/test_skeleton.c
index 77ae86f44db5..374ccef704e1 100644
--- a/tools/testing/selftests/bpf/progs/test_skeleton.c
+++ b/tools/testing/selftests/bpf/progs/test_skeleton.c
@@ -20,7 +20,9 @@ long long in4 __attribute__((aligned(64))) = 0;
 struct s in5 = {};
 
 /* .rodata section */
-const volatile int in6 = 0;
+const volatile struct {
+	const int in6;
+} in = {};
 
 /* .data section */
 int out1 = -1;
@@ -46,7 +48,7 @@ int handler(const void *ctx)
 	out3 = in3;
 	out4 = in4;
 	out5 = in5;
-	out6 = in6;
+	out6 = in.in6;
 
 	bpf_syscall = CONFIG_BPF_SYSCALL;
 	kern_ver = LINUX_KERNEL_VERSION;
-- 
2.24.1

Re: [PATCH v2 bpf-next 0/2] Strip away modifiers from BPF skeleton global variables

[Alexei Starovoitov]

On Mon, Jul 13, 2020 at 4:25 PM Andrii Nakryiko <andriin@fb.com> wrote:
>
> Fix bpftool logic of stripping away const/volatile modifiers for all global
> variables during BPF skeleton generation. See patch #1 for details on when
> existing logic breaks and why it's important. Support special .strip_mods=true
> mode in btf_dump__emit_type_decl.
>
> Recent example of when this has caused problems can be found in [0].
>
>   [0] https://github.com/iovisor/bcc/pull/2994#issuecomment-650588533

Applied. Thanks