bpf: Support llvm-objcopy for vmlinux BTF

commit 90ceddcb495008ac8ba7a3dce297841efcd7d584 upstream.

Simplify gen_btf logic to make it work with llvm-objcopy. The existing
'file format' and 'architecture' parsing logic is brittle and does not
work with llvm-objcopy/llvm-objdump.

'file format' output of llvm-objdump>=11 will match GNU objdump, but
'architecture' (bfdarch) may not.

.BTF in .tmp_vmlinux.btf is non-SHF_ALLOC. Add the SHF_ALLOC flag
because it is part of vmlinux image used for introspection. C code
can reference the section via linker script defined __start_BTF and
__stop_BTF. This fixes a small problem that previous .BTF had the
SHF_WRITE flag (objcopy -I binary -O elf* synthesized .data).

Additionally, `objcopy -I binary` synthesized symbols
_binary__btf_vmlinux_bin_start and _binary__btf_vmlinux_bin_stop (not
used elsewhere) are replaced with more commonplace __start_BTF and
__stop_BTF.

Add 2>/dev/null because GNU objcopy (but not llvm-objcopy) warns
"empty loadable segment detected at vaddr=0xffffffff81000000, is this intentional?"

We use a dd command to change the e_type field in the ELF header from
ET_EXEC to ET_REL so that lld will accept .btf.vmlinux.bin.o.  Accepting
ET_EXEC as an input file is an extremely rare GNU ld feature that lld
does not intend to support, because this is error-prone.

The output section description .BTF in include/asm-generic/vmlinux.lds.h
avoids potential subtle orphan section placement issues and suppresses
--orphan-handling=warn warnings.

Fixes: df786c9b9476 ("bpf: Force .BTF section start to zero when dumping from vmlinux")
Fixes: cb0cc635c7a9 ("powerpc: Include .BTF section")
Reported-by: Nathan Chancellor <natechancellor@gmail.com>
Signed-off-by: Fangrui Song <maskray@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Tested-by: Stanislav Fomichev <sdf@google.com>
Tested-by: Andrii Nakryiko <andriin@fb.com>
Reviewed-by: Stanislav Fomichev <sdf@google.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Acked-by: Michael Ellerman <mpe@ellerman.id.au> (powerpc)
Link: https://github.com/ClangBuiltLinux/linux/issues/871
Link: https://lore.kernel.org/bpf/20200318222746.173648-1-maskray@google.com
Signed-off-by: Maria Teguiani <teguiani@google.com>
Tested-by: Matthias Maennich <maennich@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/arch/powerpc/kernel/vmlinux.lds.S b/arch/powerpc/kernel/vmlinux.lds.S
index 4638d28..060a1ac 100644 (file)
--- a/arch/powerpc/kernel/vmlinux.lds.S
+++ b/arch/powerpc/kernel/vmlinux.lds.S
@@ -326,12 +326,6 @@ SECTIONS
                *(.branch_lt)
        }
 
-#ifdef CONFIG_DEBUG_INFO_BTF
-       .BTF : AT(ADDR(.BTF) - LOAD_OFFSET) {
-               *(.BTF)
-       }
-#endif
-
        .opd : AT(ADDR(.opd) - LOAD_OFFSET) {
                __start_opd = .;
                KEEP(*(.opd))

diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
index dae6460..b6d7347 100644 (file)
--- a/include/asm-generic/vmlinux.lds.h
+++ b/include/asm-generic/vmlinux.lds.h
@@ -496,10 +496,12 @@
                __start___modver = .;                                   \
                KEEP(*(__modver))                                       \
                __stop___modver = .;                                    \
-               . = ALIGN((align));                                     \
-               __end_rodata = .;                                       \
        }                                                               \
-       . = ALIGN((align));
+                                                                       \
+       BTF                                                             \
+                                                                       \
+       . = ALIGN((align));                                             \
+       __end_rodata = .;
 
 /* RODATA & RO_DATA provided for backward compatibility.
  * All archs are supposed to use RO_DATA() */
@@ -589,6 +591,20 @@
        }
 
 /*
+ * .BTF
+ */
+#ifdef CONFIG_DEBUG_INFO_BTF
+#define BTF                                                            \
+       .BTF : AT(ADDR(.BTF) - LOAD_OFFSET) {                           \
+               __start_BTF = .;                                        \
+               *(.BTF)                                                 \
+               __stop_BTF = .;                                         \
+       }
+#else
+#define BTF
+#endif
+
+/*
  * Init task
  */
 #define INIT_TASK_DATA_SECTION(align)                                  \

diff --git a/kernel/bpf/sysfs_btf.c b/kernel/bpf/sysfs_btf.c
index 7ae5ddd..3b49577 100644 (file)
--- a/kernel/bpf/sysfs_btf.c
+++ b/kernel/bpf/sysfs_btf.c
@@ -9,15 +9,15 @@
 #include <linux/sysfs.h>
 
 /* See scripts/link-vmlinux.sh, gen_btf() func for details */
-extern char __weak _binary__btf_vmlinux_bin_start[];
-extern char __weak _binary__btf_vmlinux_bin_end[];
+extern char __weak __start_BTF[];
+extern char __weak __stop_BTF[];
 
 static ssize_t
 btf_vmlinux_read(struct file *file, struct kobject *kobj,
                 struct bin_attribute *bin_attr,
                 char *buf, loff_t off, size_t len)
 {
-       memcpy(buf, _binary__btf_vmlinux_bin_start + off, len);
+       memcpy(buf, __start_BTF + off, len);
        return len;
 }
 
@@ -30,15 +30,14 @@ static struct kobject *btf_kobj;
 
 static int __init btf_vmlinux_init(void)
 {
-       if (!_binary__btf_vmlinux_bin_start)
+       if (!__start_BTF)
                return 0;
 
        btf_kobj = kobject_create_and_add("btf", kernel_kobj);
        if (!btf_kobj)
                return -ENOMEM;
 
-       bin_attr_btf_vmlinux.size = _binary__btf_vmlinux_bin_end -
-                                   _binary__btf_vmlinux_bin_start;
+       bin_attr_btf_vmlinux.size = __stop_BTF - __start_BTF;
 
        return sysfs_create_bin_file(btf_kobj, &bin_attr_btf_vmlinux);
 }

diff --git a/scripts/link-vmlinux.sh b/scripts/link-vmlinux.sh
index aa13860..8b6325c 100755 (executable)
--- a/scripts/link-vmlinux.sh
+++ b/scripts/link-vmlinux.sh
@@ -113,9 +113,6 @@ vmlinux_link()
 gen_btf()
 {
        local pahole_ver
-       local bin_arch
-       local bin_format
-       local bin_file
 
        if ! [ -x "$(command -v ${PAHOLE})" ]; then
                echo >&2 "BTF: ${1}: pahole (${PAHOLE}) is not available"
@@ -133,17 +130,16 @@ gen_btf()
        info "BTF" ${2}
        LLVM_OBJCOPY=${OBJCOPY} ${PAHOLE} -J ${1}
 
-       # dump .BTF section into raw binary file to link with final vmlinux
-       bin_arch=$(LANG=C ${OBJDUMP} -f ${1} | grep architecture | \
-               cut -d, -f1 | cut -d' ' -f2)
-       bin_format=$(LANG=C ${OBJDUMP} -f ${1} | grep 'file format' | \
-               awk '{print $4}')
-       bin_file=.btf.vmlinux.bin
-       ${OBJCOPY} --change-section-address .BTF=0 \
-               --set-section-flags .BTF=alloc -O binary \
-               --only-section=.BTF ${1} $bin_file
-       ${OBJCOPY} -I binary -O ${bin_format} -B ${bin_arch} \
-               --rename-section .data=.BTF $bin_file ${2}
+       # Create ${2} which contains just .BTF section but no symbols. Add
+       # SHF_ALLOC because .BTF will be part of the vmlinux image. --strip-all
+       # deletes all symbols including __start_BTF and __stop_BTF, which will
+       # be redefined in the linker script. Add 2>/dev/null to suppress GNU
+       # objcopy warnings: "empty loadable segment detected at ..."
+       ${OBJCOPY} --only-section=.BTF --set-section-flags .BTF=alloc,readonly \
+               --strip-all ${1} ${2} 2>/dev/null
+       # Change e_type to ET_REL so that it can be used to link final vmlinux.
+       # Unlike GNU ld, lld does not allow an ET_EXEC input.
+       printf '\1' | dd of=${2} conv=notrunc bs=1 seek=16 status=none
 }
 
 # Create ${2} .o file with all symbols from the ${1} object file