diff mbox series

[bpf-next,v4,6/8] libbpf: Support linking bpf objects of either endianness

Message ID 895753fe6de5a06dee8ac110f0c05571db73e70c.1724976539.git.tony.ambardar@gmail.com (mailing list archive)
State Superseded
Delegated to: BPF
Headers show
Series libbpf, selftests/bpf: Support cross-endian usage | expand

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Commit Message

Tony Ambardar Aug. 30, 2024, 7:29 a.m. UTC 
Allow static linking object files of either endianness, checking that input
files have consistent byte-order, and setting output endianness from input.

Linking requires in-memory processing of programs, relocations, sections,
etc. in native endianness, and output conversion to target byte-order. This
is enabled by built-in ELF translation and recent BTF/BTF.ext endianness
functions. Further add local functions for swapping byte-order of sections
containing BPF insns.

Signed-off-by: Tony Ambardar <tony.ambardar@gmail.com>
---
 tools/lib/bpf/linker.c | 90 ++++++++++++++++++++++++++++++++++--------
 1 file changed, 74 insertions(+), 16 deletions(-)

Comments

Andrii Nakryiko Aug. 30, 2024, 9:25 p.m. UTC | #1 
On Fri, Aug 30, 2024 at 12:30 AM Tony Ambardar <tony.ambardar@gmail.com> wrote:
>
> Allow static linking object files of either endianness, checking that input
> files have consistent byte-order, and setting output endianness from input.
>
> Linking requires in-memory processing of programs, relocations, sections,
> etc. in native endianness, and output conversion to target byte-order. This
> is enabled by built-in ELF translation and recent BTF/BTF.ext endianness
> functions. Further add local functions for swapping byte-order of sections
> containing BPF insns.
>
> Signed-off-by: Tony Ambardar <tony.ambardar@gmail.com>
> ---
>  tools/lib/bpf/linker.c | 90 ++++++++++++++++++++++++++++++++++--------
>  1 file changed, 74 insertions(+), 16 deletions(-)
>

[...]

>
> +static bool is_exec_sec(struct dst_sec *sec)
> +{
> +       if (!sec || sec->ephemeral)
> +               return false;
> +       return (sec->shdr->sh_type == SHT_PROGBITS) &&
> +              (sec->shdr->sh_flags & SHF_EXECINSTR);
> +}
> +
> +static int exec_sec_bswap(void *raw_data, int size)
> +{
> +       const int insn_cnt = size / sizeof(struct bpf_insn);
> +       struct bpf_insn *insn = raw_data;
> +       int i;
> +
> +       if (size % sizeof(struct bpf_insn))
> +               return -EINVAL;

this shouldn't be checked here, it should be assumed this is valid and
was ensured by the caller. And make exec_sec_bswap() a void function,
please.

> +       for (i = 0; i < insn_cnt; i++, insn++)
> +               bpf_insn_bswap(insn);
> +       return 0;
> +}
> +
>  static int extend_sec(struct bpf_linker *linker, struct dst_sec *dst, struct src_sec *src)
>  {
>         void *tmp;
> @@ -1170,6 +1203,10 @@ static int extend_sec(struct bpf_linker *linker, struct dst_sec *dst, struct src
>                 memset(dst->raw_data + dst->sec_sz, 0, dst_align_sz - dst->sec_sz);
>                 /* now copy src data at a properly aligned offset */
>                 memcpy(dst->raw_data + dst_align_sz, src->data->d_buf, src->shdr->sh_size);
> +

the check for size % sizeof(struct bpf_insn) should be somewhere here
(if is_exec_sec()), right?

> +               /* convert added bpf insns to native byte-order */
> +               if (linker->swapped_endian && is_exec_sec(dst))
> +                       exec_sec_bswap(dst->raw_data + dst_align_sz, src->shdr->sh_size);
>         }
>
>         dst->sec_sz = dst_final_sz;
> @@ -2630,6 +2667,10 @@ int bpf_linker__finalize(struct bpf_linker *linker)
>                 if (!sec->scn)
>                         continue;
>

but no need to check here, we know it's correct, if we got all the way here

> +               /* restore sections with bpf insns to target byte-order */
> +               if (linker->swapped_endian && is_exec_sec(sec))
> +                       exec_sec_bswap(sec->raw_data, sec->sec_sz);
> +
>                 sec->data->d_buf = sec->raw_data;
>         }
>
> @@ -2698,6 +2739,7 @@ static int emit_elf_data_sec(struct bpf_linker *linker, const char *sec_name,
>
>  static int finalize_btf(struct bpf_linker *linker)
>  {
> +       enum btf_endianness link_endianness;
>         LIBBPF_OPTS(btf_dedup_opts, opts);
>         struct btf *btf = linker->btf;
>         const void *raw_data;
> @@ -2742,6 +2784,22 @@ static int finalize_btf(struct bpf_linker *linker)
>                 return err;
>         }
>
> +       /* Set .BTF and .BTF.ext output byte order */
> +       link_endianness = linker->elf_hdr->e_ident[EI_DATA] == ELFDATA2MSB ?
> +                         BTF_BIG_ENDIAN : BTF_LITTLE_ENDIAN;
> +       err = btf__set_endianness(linker->btf, link_endianness);
> +       if (err) {
> +               pr_warn("failed to set .BTF output endianness: %d\n", err);
> +               return err;
> +       }

link_endianness is always well-formed enum, there is no need to check
errors, here and for btf_ext__set_endianness, please drop both

> +       if (linker->btf_ext) {
> +               err = btf_ext__set_endianness(linker->btf_ext, link_endianness);
> +               if (err) {
> +                       pr_warn("failed to set .BTF.ext output endianness: %d\n", err);
> +                       return err;
> +               }
> +       }
> +
>         /* Emit .BTF section */
>         raw_data = btf__raw_data(linker->btf, &raw_sz);
>         if (!raw_data)
> --
> 2.34.1
>
Tony Ambardar Sept. 1, 2024, 6:02 a.m. UTC | #2 
On Fri, Aug 30, 2024 at 02:25:07PM -0700, Andrii Nakryiko wrote:
> On Fri, Aug 30, 2024 at 12:30 AM Tony Ambardar <tony.ambardar@gmail.com> wrote:
> >
> > Allow static linking object files of either endianness, checking that input
> > files have consistent byte-order, and setting output endianness from input.
> >
> > Linking requires in-memory processing of programs, relocations, sections,
> > etc. in native endianness, and output conversion to target byte-order. This
> > is enabled by built-in ELF translation and recent BTF/BTF.ext endianness
> > functions. Further add local functions for swapping byte-order of sections
> > containing BPF insns.
> >
> > Signed-off-by: Tony Ambardar <tony.ambardar@gmail.com>
> > ---
> >  tools/lib/bpf/linker.c | 90 ++++++++++++++++++++++++++++++++++--------
> >  1 file changed, 74 insertions(+), 16 deletions(-)
> >
> 
> [...]
> 
> >
> > +static bool is_exec_sec(struct dst_sec *sec)
> > +{
> > +       if (!sec || sec->ephemeral)
> > +               return false;
> > +       return (sec->shdr->sh_type == SHT_PROGBITS) &&
> > +              (sec->shdr->sh_flags & SHF_EXECINSTR);
> > +}
> > +
> > +static int exec_sec_bswap(void *raw_data, int size)
> > +{
> > +       const int insn_cnt = size / sizeof(struct bpf_insn);
> > +       struct bpf_insn *insn = raw_data;
> > +       int i;
> > +
> > +       if (size % sizeof(struct bpf_insn))
> > +               return -EINVAL;
> 
> this shouldn't be checked here, it should be assumed this is valid and
> was ensured by the caller. And make exec_sec_bswap() a void function,
> please.
> 
> > +       for (i = 0; i < insn_cnt; i++, insn++)
> > +               bpf_insn_bswap(insn);
> > +       return 0;
> > +}
> > +
> >  static int extend_sec(struct bpf_linker *linker, struct dst_sec *dst, struct src_sec *src)
> >  {
> >         void *tmp;
> > @@ -1170,6 +1203,10 @@ static int extend_sec(struct bpf_linker *linker, struct dst_sec *dst, struct src
> >                 memset(dst->raw_data + dst->sec_sz, 0, dst_align_sz - dst->sec_sz);
> >                 /* now copy src data at a properly aligned offset */
> >                 memcpy(dst->raw_data + dst_align_sz, src->data->d_buf, src->shdr->sh_size);
> > +
> 
> the check for size % sizeof(struct bpf_insn) should be somewhere here
> (if is_exec_sec()), right?
> 
> > +               /* convert added bpf insns to native byte-order */
> > +               if (linker->swapped_endian && is_exec_sec(dst))
> > +                       exec_sec_bswap(dst->raw_data + dst_align_sz, src->shdr->sh_size);
> >         }
> >
> >         dst->sec_sz = dst_final_sz;
> > @@ -2630,6 +2667,10 @@ int bpf_linker__finalize(struct bpf_linker *linker)
> >                 if (!sec->scn)
> >                         continue;
> >
> 
> but no need to check here, we know it's correct, if we got all the way here
> 

I did a pass earlier to reorganize sanity checks and remove redundant
ones, and realized linker_sanity_check_elf() already does what we want
but overlooked dropping this from exec_sec_bswap(). Will do so now. Thanks
for catching!

> > +               /* restore sections with bpf insns to target byte-order */
> > +               if (linker->swapped_endian && is_exec_sec(sec))
> > +                       exec_sec_bswap(sec->raw_data, sec->sec_sz);
> > +
> >                 sec->data->d_buf = sec->raw_data;
> >         }
> >
> > @@ -2698,6 +2739,7 @@ static int emit_elf_data_sec(struct bpf_linker *linker, const char *sec_name,
> >
> >  static int finalize_btf(struct bpf_linker *linker)
> >  {
> > +       enum btf_endianness link_endianness;
> >         LIBBPF_OPTS(btf_dedup_opts, opts);
> >         struct btf *btf = linker->btf;
> >         const void *raw_data;
> > @@ -2742,6 +2784,22 @@ static int finalize_btf(struct bpf_linker *linker)
> >                 return err;
> >         }
> >
> > +       /* Set .BTF and .BTF.ext output byte order */
> > +       link_endianness = linker->elf_hdr->e_ident[EI_DATA] == ELFDATA2MSB ?
> > +                         BTF_BIG_ENDIAN : BTF_LITTLE_ENDIAN;
> > +       err = btf__set_endianness(linker->btf, link_endianness);
> > +       if (err) {
> > +               pr_warn("failed to set .BTF output endianness: %d\n", err);
> > +               return err;
> > +       }
> 
> link_endianness is always well-formed enum, there is no need to check
> errors, here and for btf_ext__set_endianness, please drop both

Right, makes sense.

> 
> > +       if (linker->btf_ext) {
> > +               err = btf_ext__set_endianness(linker->btf_ext, link_endianness);
> > +               if (err) {
> > +                       pr_warn("failed to set .BTF.ext output endianness: %d\n", err);
> > +                       return err;
> > +               }
> > +       }
> > +
> >         /* Emit .BTF section */
> >         raw_data = btf__raw_data(linker->btf, &raw_sz);
> >         if (!raw_data)
> > --
> > 2.34.1
> >
diff mbox series

Patch

diff --git a/tools/lib/bpf/linker.c b/tools/lib/bpf/linker.c
index 7489306cd6f7..bd97da68eed6 100644
--- a/tools/lib/bpf/linker.c
+++ b/tools/lib/bpf/linker.c
@@ -135,6 +135,7 @@  struct bpf_linker {
 	int fd;
 	Elf *elf;
 	Elf64_Ehdr *elf_hdr;
+	bool swapped_endian;
 
 	/* Output sections metadata */
 	struct dst_sec *secs;
@@ -324,13 +325,8 @@  static int init_output_elf(struct bpf_linker *linker, const char *file)
 
 	linker->elf_hdr->e_machine = EM_BPF;
 	linker->elf_hdr->e_type = ET_REL;
-#if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
-	linker->elf_hdr->e_ident[EI_DATA] = ELFDATA2LSB;
-#elif __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__
-	linker->elf_hdr->e_ident[EI_DATA] = ELFDATA2MSB;
-#else
-#error "Unknown __BYTE_ORDER__"
-#endif
+	/* Set unknown ELF endianness, assign later from input files */
+	linker->elf_hdr->e_ident[EI_DATA] = ELFDATANONE;
 
 	/* STRTAB */
 	/* initialize strset with an empty string to conform to ELF */
@@ -541,19 +537,21 @@  static int linker_load_obj_file(struct bpf_linker *linker, const char *filename,
 				const struct bpf_linker_file_opts *opts,
 				struct src_obj *obj)
 {
-#if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
-	const int host_endianness = ELFDATA2LSB;
-#elif __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__
-	const int host_endianness = ELFDATA2MSB;
-#else
-#error "Unknown __BYTE_ORDER__"
-#endif
 	int err = 0;
 	Elf_Scn *scn;
 	Elf_Data *data;
 	Elf64_Ehdr *ehdr;
 	Elf64_Shdr *shdr;
 	struct src_sec *sec;
+	unsigned char obj_byteorder;
+	unsigned char link_byteorder = linker->elf_hdr->e_ident[EI_DATA];
+#if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
+	const unsigned char host_byteorder = ELFDATA2LSB;
+#elif __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__
+	const unsigned char host_byteorder = ELFDATA2MSB;
+#else
+#error "Unknown __BYTE_ORDER__"
+#endif
 
 	pr_debug("linker: adding object file '%s'...\n", filename);
 
@@ -579,11 +577,25 @@  static int linker_load_obj_file(struct bpf_linker *linker, const char *filename,
 		pr_warn_elf("failed to get ELF header for %s", filename);
 		return err;
 	}
-	if (ehdr->e_ident[EI_DATA] != host_endianness) {
+
+	/* Linker output endianness set by first input object */
+	obj_byteorder = ehdr->e_ident[EI_DATA];
+	if (obj_byteorder != ELFDATA2LSB && obj_byteorder != ELFDATA2MSB) {
+		err = -EOPNOTSUPP;
+		pr_warn("unknown byte order of ELF file %s\n", filename);
+		return err;
+	}
+	if (link_byteorder == ELFDATANONE) {
+		linker->elf_hdr->e_ident[EI_DATA] = obj_byteorder;
+		linker->swapped_endian = obj_byteorder != host_byteorder;
+		pr_debug("linker: set %s-endian output byte order\n",
+			 obj_byteorder == ELFDATA2MSB ? "big" : "little");
+	} else if (link_byteorder != obj_byteorder) {
 		err = -EOPNOTSUPP;
-		pr_warn_elf("unsupported byte order of ELF file %s", filename);
+		pr_warn("byte order mismatch with ELF file %s\n", filename);
 		return err;
 	}
+
 	if (ehdr->e_type != ET_REL
 	    || ehdr->e_machine != EM_BPF
 	    || ehdr->e_ident[EI_CLASS] != ELFCLASS64) {
@@ -1111,6 +1123,27 @@  static bool sec_content_is_same(struct dst_sec *dst_sec, struct src_sec *src_sec
 	return true;
 }
 
+static bool is_exec_sec(struct dst_sec *sec)
+{
+	if (!sec || sec->ephemeral)
+		return false;
+	return (sec->shdr->sh_type == SHT_PROGBITS) &&
+	       (sec->shdr->sh_flags & SHF_EXECINSTR);
+}
+
+static int exec_sec_bswap(void *raw_data, int size)
+{
+	const int insn_cnt = size / sizeof(struct bpf_insn);
+	struct bpf_insn *insn = raw_data;
+	int i;
+
+	if (size % sizeof(struct bpf_insn))
+		return -EINVAL;
+	for (i = 0; i < insn_cnt; i++, insn++)
+		bpf_insn_bswap(insn);
+	return 0;
+}
+
 static int extend_sec(struct bpf_linker *linker, struct dst_sec *dst, struct src_sec *src)
 {
 	void *tmp;
@@ -1170,6 +1203,10 @@  static int extend_sec(struct bpf_linker *linker, struct dst_sec *dst, struct src
 		memset(dst->raw_data + dst->sec_sz, 0, dst_align_sz - dst->sec_sz);
 		/* now copy src data at a properly aligned offset */
 		memcpy(dst->raw_data + dst_align_sz, src->data->d_buf, src->shdr->sh_size);
+
+		/* convert added bpf insns to native byte-order */
+		if (linker->swapped_endian && is_exec_sec(dst))
+			exec_sec_bswap(dst->raw_data + dst_align_sz, src->shdr->sh_size);
 	}
 
 	dst->sec_sz = dst_final_sz;
@@ -2630,6 +2667,10 @@  int bpf_linker__finalize(struct bpf_linker *linker)
 		if (!sec->scn)
 			continue;
 
+		/* restore sections with bpf insns to target byte-order */
+		if (linker->swapped_endian && is_exec_sec(sec))
+			exec_sec_bswap(sec->raw_data, sec->sec_sz);
+
 		sec->data->d_buf = sec->raw_data;
 	}
 
@@ -2698,6 +2739,7 @@  static int emit_elf_data_sec(struct bpf_linker *linker, const char *sec_name,
 
 static int finalize_btf(struct bpf_linker *linker)
 {
+	enum btf_endianness link_endianness;
 	LIBBPF_OPTS(btf_dedup_opts, opts);
 	struct btf *btf = linker->btf;
 	const void *raw_data;
@@ -2742,6 +2784,22 @@  static int finalize_btf(struct bpf_linker *linker)
 		return err;
 	}
 
+	/* Set .BTF and .BTF.ext output byte order */
+	link_endianness = linker->elf_hdr->e_ident[EI_DATA] == ELFDATA2MSB ?
+			  BTF_BIG_ENDIAN : BTF_LITTLE_ENDIAN;
+	err = btf__set_endianness(linker->btf, link_endianness);
+	if (err) {
+		pr_warn("failed to set .BTF output endianness: %d\n", err);
+		return err;
+	}
+	if (linker->btf_ext) {
+		err = btf_ext__set_endianness(linker->btf_ext, link_endianness);
+		if (err) {
+			pr_warn("failed to set .BTF.ext output endianness: %d\n", err);
+			return err;
+		}
+	}
+
 	/* Emit .BTF section */
 	raw_data = btf__raw_data(linker->btf, &raw_sz);
 	if (!raw_data)