tools: bpftool: Restore message on failure to guess program type

In commit 4a3d6c6a6e ("libbpf: Reduce log level for custom section
names"), log level for messages for libbpf_attach_type_by_name() and
libbpf_prog_type_by_name() was downgraded from "info" to "debug". The
latter function, in particular, is used by bpftool when attempting to
load programs, and this change caused bpftool to exit with no hint or
error message when it fails to detect the type of the program to load
(unless "-d" option was provided).

To help users understand why bpftool fails to load the program, let's do
a second run of the function with log level in "debug" mode in case of
failure.

Before:

    # bpftool prog load sample_ret0.o /sys/fs/bpf/sample_ret0
    # echo $?
    255

Or really verbose with -d flag:

    # bpftool -d prog load sample_ret0.o /sys/fs/bpf/sample_ret0
    libbpf: loading sample_ret0.o
    libbpf: section(1) .strtab, size 134, link 0, flags 0, type=3
    libbpf: skip section(1) .strtab
    libbpf: section(2) .text, size 16, link 0, flags 6, type=1
    libbpf: found program .text
    libbpf: section(3) .debug_abbrev, size 55, link 0, flags 0, type=1
    libbpf: skip section(3) .debug_abbrev
    libbpf: section(4) .debug_info, size 75, link 0, flags 0, type=1
    libbpf: skip section(4) .debug_info
    libbpf: section(5) .rel.debug_info, size 32, link 14, flags 0, type=9
    libbpf: skip relo .rel.debug_info(5) for section(4)
    libbpf: section(6) .debug_str, size 150, link 0, flags 30, type=1
    libbpf: skip section(6) .debug_str
    libbpf: section(7) .BTF, size 155, link 0, flags 0, type=1
    libbpf: section(8) .BTF.ext, size 80, link 0, flags 0, type=1
    libbpf: section(9) .rel.BTF.ext, size 32, link 14, flags 0, type=9
    libbpf: skip relo .rel.BTF.ext(9) for section(8)
    libbpf: section(10) .debug_frame, size 40, link 0, flags 0, type=1
    libbpf: skip section(10) .debug_frame
    libbpf: section(11) .rel.debug_frame, size 16, link 14, flags 0, type=9
    libbpf: skip relo .rel.debug_frame(11) for section(10)
    libbpf: section(12) .debug_line, size 74, link 0, flags 0, type=1
    libbpf: skip section(12) .debug_line
    libbpf: section(13) .rel.debug_line, size 16, link 14, flags 0, type=9
    libbpf: skip relo .rel.debug_line(13) for section(12)
    libbpf: section(14) .symtab, size 96, link 1, flags 0, type=2
    libbpf: looking for externs among 4 symbols...
    libbpf: collected 0 externs total
    libbpf: failed to guess program type from ELF section '.text'
    libbpf: supported section(type) names are: socket sk_reuseport kprobe/ [...]

After:

    # bpftool prog load sample_ret0.o /sys/fs/bpf/sample_ret0
    libbpf: failed to guess program type from ELF section '.text'
    libbpf: supported section(type) names are: socket sk_reuseport kprobe/ [...]

Signed-off-by: Quentin Monnet <quentin@isovalent.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/20200311021205.9755-1-quentin@isovalent.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

tools/bpf/bpftool/common.c

@@ -572,3 +572,10 @@ int parse_u32_arg(int *argc, char ***argv, __u32 *val, const char *what)
 
 	return 0;
 }
+
+int __printf(2, 0)
+print_all_levels(__maybe_unused enum libbpf_print_level level,
+		 const char *format, va_list args)
+{
+	return vfprintf(stderr, format, args);
+}

tools/bpf/bpftool/main.c

@@ -79,13 +79,6 @@ static int do_version(int argc, char **argv)
 	return 0;
 }
 
-static int __printf(2, 0)
-print_all_levels(__maybe_unused enum libbpf_print_level level,
-		 const char *format, va_list args)
-{
-	return vfprintf(stderr, format, args);
-}
-
 int cmd_select(const struct cmd *cmds, int argc, char **argv,
 	       int (*help)(int argc, char **argv))
 {

tools/bpf/bpftool/main.h

@@ -14,6 +14,8 @@
 #include <linux/hashtable.h>
 #include <tools/libc_compat.h>
 
+#include <bpf/libbpf.h>
+
 #include "json_writer.h"
 
 #define ptr_to_u64(ptr)	((__u64)(unsigned long)(ptr))
@@ -229,4 +231,7 @@ struct tcmsg;
 int do_xdp_dump(struct ifinfomsg *ifinfo, struct nlattr **tb);
 int do_filter_dump(struct tcmsg *ifinfo, struct nlattr **tb, const char *kind,
 		   const char *devname, int ifindex);
+
+int print_all_levels(__maybe_unused enum libbpf_print_level level,
+		     const char *format, va_list args);
 #endif

tools/bpf/bpftool/prog.c

@@ -1247,6 +1247,25 @@ free_data_in:
 	return err;
 }
 
+static int
+get_prog_type_by_name(const char *name, enum bpf_prog_type *prog_type,
+		      enum bpf_attach_type *expected_attach_type)
+{
+	libbpf_print_fn_t print_backup;
+	int ret;
+
+	ret = libbpf_prog_type_by_name(name, prog_type, expected_attach_type);
+	if (!ret)
+		return ret;
+
+	/* libbpf_prog_type_by_name() failed, let's re-run with debug level */
+	print_backup = libbpf_set_print(print_all_levels);
+	ret = libbpf_prog_type_by_name(name, prog_type, expected_attach_type);
+	libbpf_set_print(print_backup);
+
+	return ret;
+}
+
 static int load_with_options(int argc, char **argv, bool first_prog_only)
 {
 	enum bpf_prog_type common_prog_type = BPF_PROG_TYPE_UNSPEC;
@@ -1296,8 +1315,8 @@ static int load_with_options(int argc, char **argv, bool first_prog_only)
 			strcat(type, *argv);
 			strcat(type, "/");
 
-			err = libbpf_prog_type_by_name(type, &common_prog_type,
-						       &expected_attach_type);
+			err = get_prog_type_by_name(type, &common_prog_type,
+						    &expected_attach_type);
 			free(type);
 			if (err < 0)
 				goto err_free_reuse_maps;
@@ -1396,8 +1415,8 @@ static int load_with_options(int argc, char **argv, bool first_prog_only)
 		if (prog_type == BPF_PROG_TYPE_UNSPEC) {
 			const char *sec_name = bpf_program__title(pos, false);
 
-			err = libbpf_prog_type_by_name(sec_name, &prog_type,
-						       &expected_attach_type);
+			err = get_prog_type_by_name(sec_name, &prog_type,
+						    &expected_attach_type);
 			if (err < 0)
 				goto err_close_obj;
 		}