[PATCH bpf-next] libbpf: provide more helpful message on uninitialized global var

[PATCH bpf-next] libbpf: provide more helpful message on uninitialized global var

[Andrii Nakryiko]

When BPF program defines uninitialized global variable, it's put into
a special COMMON section. Libbpf will reject such programs, but will
provide very unhelpful message with garbage-looking section index.

This patch detects special section cases and gives more explicit error
message.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
---
 tools/lib/bpf/libbpf.c | 14 +++++++++++---
 1 file changed, 11 insertions(+), 3 deletions(-)

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index 794dd5064ae8..5f9e7eedb134 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -1760,15 +1760,23 @@ bpf_program__collect_reloc(struct bpf_program *prog, GElf_Shdr *shdr,
 			 (long long) sym.st_value, sym.st_name, name);
 
 		shdr_idx = sym.st_shndx;
+		insn_idx = rel.r_offset / sizeof(struct bpf_insn);
+		pr_debug("relocation: insn_idx=%u, shdr_idx=%u\n",
+			 insn_idx, shdr_idx);
+
+		if (shdr_idx >= SHN_LORESERVE) {
+			pr_warning("relocation: not yet supported relo for non-static global \'%s\' variable "
+				   "in special section (0x%x) found in insns[%d].code 0x%x\n",
+				   name, shdr_idx, insn_idx,
+				   insns[insn_idx].code);
+			return -LIBBPF_ERRNO__RELOC;
+		}
 		if (!bpf_object__relo_in_known_section(obj, shdr_idx)) {
 			pr_warning("Program '%s' contains unrecognized relo data pointing to section %u\n",
 				   prog->section_name, shdr_idx);
 			return -LIBBPF_ERRNO__RELOC;
 		}
 
-		insn_idx = rel.r_offset / sizeof(struct bpf_insn);
-		pr_debug("relocation: insn_idx=%u\n", insn_idx);
-
 		if (insns[insn_idx].code == (BPF_JMP | BPF_CALL)) {
 			if (insns[insn_idx].src_reg != BPF_PSEUDO_CALL) {
 				pr_warning("incorrect bpf_call opcode\n");
-- 
2.17.1

Re: [PATCH bpf-next] libbpf: provide more helpful message on uninitialized global var

[Song Liu]

> On Jul 22, 2019, at 9:23 PM, Andrii Nakryiko <andriin@fb.com> wrote:
> 
> When BPF program defines uninitialized global variable, it's put into
> a special COMMON section. Libbpf will reject such programs, but will
> provide very unhelpful message with garbage-looking section index.
> 
> This patch detects special section cases and gives more explicit error
> message.
> 
> Signed-off-by: Andrii Nakryiko <andriin@fb.com>
> ---
> tools/lib/bpf/libbpf.c | 14 +++++++++++---
> 1 file changed, 11 insertions(+), 3 deletions(-)
> 
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index 794dd5064ae8..5f9e7eedb134 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -1760,15 +1760,23 @@ bpf_program__collect_reloc(struct bpf_program *prog, GElf_Shdr *shdr,
> 			 (long long) sym.st_value, sym.st_name, name);
> 
> 		shdr_idx = sym.st_shndx;
> +		insn_idx = rel.r_offset / sizeof(struct bpf_insn);
> +		pr_debug("relocation: insn_idx=%u, shdr_idx=%u\n",
> +			 insn_idx, shdr_idx);
> +
> +		if (shdr_idx >= SHN_LORESERVE) {
> +			pr_warning("relocation: not yet supported relo for non-static global \'%s\' variable "
> +				   "in special section (0x%x) found in insns[%d].code 0x%x\n",

For easy grep, we should keep this one long long string. 

Thanks,
Song

Re: [PATCH bpf-next] libbpf: provide more helpful message on uninitialized global var

[Andrii Nakryiko]

On Tue, Jul 23, 2019 at 1:51 AM Song Liu <songliubraving@fb.com> wrote:
>
>
>
> > On Jul 22, 2019, at 9:23 PM, Andrii Nakryiko <andriin@fb.com> wrote:
> >
> > When BPF program defines uninitialized global variable, it's put into
> > a special COMMON section. Libbpf will reject such programs, but will
> > provide very unhelpful message with garbage-looking section index.
> >
> > This patch detects special section cases and gives more explicit error
> > message.
> >
> > Signed-off-by: Andrii Nakryiko <andriin@fb.com>
> > ---
> > tools/lib/bpf/libbpf.c | 14 +++++++++++---
> > 1 file changed, 11 insertions(+), 3 deletions(-)
> >
> > diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> > index 794dd5064ae8..5f9e7eedb134 100644
> > --- a/tools/lib/bpf/libbpf.c
> > +++ b/tools/lib/bpf/libbpf.c
> > @@ -1760,15 +1760,23 @@ bpf_program__collect_reloc(struct bpf_program *prog, GElf_Shdr *shdr,
> >                        (long long) sym.st_value, sym.st_name, name);
> >
> >               shdr_idx = sym.st_shndx;
> > +             insn_idx = rel.r_offset / sizeof(struct bpf_insn);
> > +             pr_debug("relocation: insn_idx=%u, shdr_idx=%u\n",
> > +                      insn_idx, shdr_idx);
> > +
> > +             if (shdr_idx >= SHN_LORESERVE) {
> > +                     pr_warning("relocation: not yet supported relo for non-static global \'%s\' variable "
> > +                                "in special section (0x%x) found in insns[%d].code 0x%x\n",
>
> For easy grep, we should keep this one long long string.

I was worried it's way too long, so split it. But yeah, I'll just
merge it back into single line, thanks!

>
> Thanks,
> Song