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bpf: Support llvm-objcopy for vmlinux BTF

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.

Fangrui Song committed 6 years ago

Commit 90ceddc1 parent 483d7a35 files · +34 −31Browse files

Unified · Split

5 files
  1. arch/powerpc/kernel/vmlinux.lds.S +0 −6
  2. include/asm-generic/vmlinux.lds.h +15 −0
  3. kernel/bpf/btf.c +4 −5
  4. kernel/bpf/sysfs_btf.c +5 −6
  5. scripts/link-vmlinux.sh +10 −14
arch/powerpc/kernel/vmlinux.lds.S+0−6
@@ -303,12 +303,6 @@ SECTIONS · 302 unmodified lines
303303 *(.branch_lt)
304304 }
305305
306−#ifdef CONFIG_DEBUG_INFO_BTF
307− .BTF : AT(ADDR(.BTF) - LOAD_OFFSET) {
308− *(.BTF)
309− }
310−#endif
311−
312306 .opd : AT(ADDR(.opd) - LOAD_OFFSET) {
313307 __start_opd = .;
314308 KEEP(*(.opd))
include/asm-generic/vmlinux.lds.h+15−0
@@ -535,6 +535,7 @@ · 534 unmodified lines
535535 \
536536 RO_EXCEPTION_TABLE \
537537 NOTES \
538+ BTF \
538539 \
539540 . = ALIGN((align)); \
540541 __end_rodata = .;
@@ -621,6 +622,20 @@ · 80 unmodified lines
621622 __stop___ex_table = .; \
622623 }
623624
625+/*
626+ * .BTF
627+ */
628+#ifdef CONFIG_DEBUG_INFO_BTF
629+#define BTF \
630+ .BTF : AT(ADDR(.BTF) - LOAD_OFFSET) { \
631+ __start_BTF = .; \
632+ *(.BTF) \
633+ __stop_BTF = .; \
634+ }
635+#else
636+#define BTF
637+#endif
638+
624639 /*
625640 * Init task
626641 */
kernel/bpf/btf.c+4−5
@@ -3477,8 +3477,8 @@ errout: · 3476 unmodified lines
34773477 return ERR_PTR(err);
34783478 }
34793479
3480−extern char __weak _binary__btf_vmlinux_bin_start[];
3481−extern char __weak _binary__btf_vmlinux_bin_end[];
3480+extern char __weak __start_BTF[];
3481+extern char __weak __stop_BTF[];
34823482 extern struct btf *btf_vmlinux;
34833483
34843484 #define BPF_MAP_TYPE(_id, _ops)
@@ -3605,9 +3605,8 @@ struct btf *btf_parse_vmlinux(void) · 120 unmodified lines
36053605 }
36063606 env->btf = btf;
36073607
3608− btf->data = _binary__btf_vmlinux_bin_start;
3609− btf->data_size = _binary__btf_vmlinux_bin_end -
3610− _binary__btf_vmlinux_bin_start;
3608+ btf->data = __start_BTF;
3609+ btf->data_size = __stop_BTF - __start_BTF;
36113610
36123611 err = btf_parse_hdr(env);
36133612 if (err)
kernel/bpf/sysfs_btf.c+5−6
@@ -9,15 +9,15 @@ · 8 unmodified lines
99 #include <linux/sysfs.h>
1010
1111 /* See scripts/link-vmlinux.sh, gen_btf() func for details */
12−extern char __weak _binary__btf_vmlinux_bin_start[];
13−extern char __weak _binary__btf_vmlinux_bin_end[];
12+extern char __weak __start_BTF[];
13+extern char __weak __stop_BTF[];
1414
1515 static ssize_t
1616 btf_vmlinux_read(struct file *file, struct kobject *kobj,
1717 struct bin_attribute *bin_attr,
1818 char *buf, loff_t off, size_t len)
1919 {
20− memcpy(buf, _binary__btf_vmlinux_bin_start + off, len);
20+ memcpy(buf, __start_BTF + off, len);
2121 return len;
2222 }
2323
@@ -30,15 +30,14 @@ static struct kobject *btf_kobj; · 6 unmodified lines
3030
3131 static int __init btf_vmlinux_init(void)
3232 {
33− if (!_binary__btf_vmlinux_bin_start)
33+ if (!__start_BTF)
3434 return 0;
3535
3636 btf_kobj = kobject_create_and_add("btf", kernel_kobj);
3737 if (!btf_kobj)
3838 return -ENOMEM;
3939
40− bin_attr_btf_vmlinux.size = _binary__btf_vmlinux_bin_end -
41− _binary__btf_vmlinux_bin_start;
40+ bin_attr_btf_vmlinux.size = __stop_BTF - __start_BTF;
4241
4342 return sysfs_create_bin_file(btf_kobj, &bin_attr_btf_vmlinux);
4443 }
scripts/link-vmlinux.sh+10−14
@@ -113,9 +113,6 @@ vmlinux_link() · 112 unmodified lines
113113 gen_btf()
114114 {
115115 local pahole_ver
116− local bin_arch
117− local bin_format
118− local bin_file
119116
120117 if ! [ -x "$(command -v ${PAHOLE})" ]; then
121118 echo >&2 "BTF: ${1}: pahole (${PAHOLE}) is not available"
@@ -133,17 +130,16 @@ gen_btf() · 11 unmodified lines
133130 info "BTF" ${2}
134131 LLVM_OBJCOPY=${OBJCOPY} ${PAHOLE} -J ${1}
135132
136− # dump .BTF section into raw binary file to link with final vmlinux
137− bin_arch=$(LANG=C ${OBJDUMP} -f ${1} | grep architecture | \
138− cut -d, -f1 | cut -d' ' -f2)
139− bin_format=$(LANG=C ${OBJDUMP} -f ${1} | grep 'file format' | \
140− awk '{print $4}')
141− bin_file=.btf.vmlinux.bin
142− ${OBJCOPY} --change-section-address .BTF=0 \
143− --set-section-flags .BTF=alloc -O binary \
144− --only-section=.BTF ${1} $bin_file
145− ${OBJCOPY} -I binary -O ${bin_format} -B ${bin_arch} \
146− --rename-section .data=.BTF $bin_file ${2}
133+ # Create ${2} which contains just .BTF section but no symbols. Add
134+ # SHF_ALLOC because .BTF will be part of the vmlinux image. --strip-all
135+ # deletes all symbols including __start_BTF and __stop_BTF, which will
136+ # be redefined in the linker script. Add 2>/dev/null to suppress GNU
137+ # objcopy warnings: "empty loadable segment detected at ..."
138+ ${OBJCOPY} --only-section=.BTF --set-section-flags .BTF=alloc,readonly \
139+ --strip-all ${1} ${2} 2>/dev/null
140+ # Change e_type to ET_REL so that it can be used to link final vmlinux.
141+ # Unlike GNU ld, lld does not allow an ET_EXEC input.
142+ printf '\1' | dd of=${2} conv=notrunc bs=1 seek=16 status=none
147143 }
148144
149145 # Create ${2} .o file with all symbols from the ${1} object file