[PATCH v5] scripts/link-vmlinux.sh: Add alias to duplicate symbols for kallsyms

Alessandro Carminati (Red Hat)
2023-09-27T17:36:14Z

It is not uncommon for drivers or modules related to similar peripherals
to have symbols with the exact same name.
While this is not a problem for the kernel's binary itself, it becomes an
issue when attempting to trace or probe specific functions using
infrastructure like ftrace or kprobe.

The tracing subsystem relies on the `nm -n vmlinux` output, which provides
symbol information from the kernel's ELF binary. However, when multiple
symbols share the same name, the standard nm output does not differentiate
between them. This can lead to confusion and difficulty when trying to
probe the intended symbol.

~ # cat /proc/kallsyms | grep " name_show"
ffffffff8c4f76d0 t name_show
ffffffff8c9cccb0 t name_show
ffffffff8cb0ac20 t name_show
ffffffff8cc728c0 t name_show
ffffffff8ce0efd0 t name_show
ffffffff8ce126c0 t name_show
ffffffff8ce1dd20 t name_show
ffffffff8ce24e70 t name_show
ffffffff8d1104c0 t name_show
ffffffff8d1fe480 t name_show

kas_alias addresses this challenge by enhancing symbol names with
meaningful suffixes generated from the source file and line number
during the kernel build process.
These newly generated aliases provide tracers with the ability to
comprehend the symbols they are interacting with when utilizing the
ftracefs interface.
This approach may also allow for the probing by name of previously
inaccessible symbols.

~ # cat /proc/kallsyms | grep gic_mask_irq
ffffd15671e505ac t gic_mask_irq
ffffd15671e505ac t gic_mask_irq@drivers_irqchip_irq_gic_c_167
ffffd15671e532a4 t gic_mask_irq
ffffd15671e532a4 t gic_mask_irq@drivers_irqchip_irq_gic_v3_c_407
~ #

Changes from v1:
- Integrated changes requested by Masami to exclude symbols with prefixes
"_cfi" and "_pfx".
- Introduced a small framework to handle patterns that need to be excluded
from the alias production.
- Excluded other symbols using the framework.
- Introduced the ability to discriminate between text and data symbols.
- Added two new config symbols in this version: CONFIG_KALLSYMS_ALIAS_DATA,
which allows data for data, and CONFIG_KALLSYMS_ALIAS_DATA_ALL, which
excludes all filters and provides an alias for each duplicated symbol.

https://lore.kernel.org/all/20230711151925.1092080-1-alessandro.carminati@gmail.com/

Changes from v2:
- Alias tags are created by querying DWARF information from the vmlinux.
- The filename + line number is normalized and appended to the original
name.
- The tag begins with '@' to indicate the symbol source.
- Not a change, but worth mentioning, since the alias is added to the
existing list, the old duplicated name is preserved, and the livepatch
way of dealing with duplicates is maintained.
- Acknowledging the existence of scenarios where inlined functions
declared in header files may result in multiple copies due to compiler
behavior, though it is not actionable as it does not pose an operational
issue.
- Highlighting a single exception where the same name refers to different
functions: the case of "compat_binfmt_elf.c," which directly includes
"binfmt_elf.c" producing identical function copies in two separate
modules.

https://lore.kernel.org/all/20230714150326.1152359-1-alessandro.carminati@gmail.com/

Changes from v3:
- kas_alias was rewritten in Python to create a more concise and
maintainable codebase.
- The previous automation process used by kas_alias to locate the vmlinux
and the addr2line has been replaced with an explicit command-line switch
for specifying these requirements.
- addr2line has been added into the main Makefile.
- A new command-line switch has been introduced, enabling users to extend
the alias to global data names.

https://lore.kernel.org/all/20230828080423.3539686-1-alessandro.carminati@gmail.com/

Changes from v4:
- Fixed the O=<build dir> build issue
- The tool halts execution upon encountering major issues, thereby ensuring
the pipeline is interrupted.
- A cmdline option to specify the source directory added.
- Minor code adjusments.
- Tested on mips32 and i386

https://lore.kernel.org/all/20230919193948.465340-1-alessandro.carminati@gmail.com/

NOTE:
About the symbols name duplication that happens as consequence of the
inclusion compat_binfmt_elf.c does, it is evident that this corner is
inherently challenging the addr2line approach.
Attempting to conceal this limitation would be counterproductive.

compat_binfmt_elf.c includes directly binfmt_elf.c, addr2line can't help
but report all functions and data declared by that file, coming from
binfmt_elf.c.

My position is that, rather than producing a more complicated pipeline
to handle this corner case, it is better to fix the compat_binfmt_elf.c
anomaly.

This patch does not deal with the two potentially problematic symbols
defined by compat_binfmt_elf.c

Signed-off-by: Alessandro Carminati (Red Hat) <alessandro.carminati@gmail.com>
---
Makefile | 4 +-
init/Kconfig | 22 +++++++
scripts/kas_alias.py | 136 ++++++++++++++++++++++++++++++++++++++++
scripts/link-vmlinux.sh | 21 ++++++-
4 files changed, 180 insertions(+), 3 deletions(-)
create mode 100755 scripts/kas_alias.py

diff --git a/Makefile b/Makefile
index 4f283d915e54..f33c179f4cc3 100644
--- a/Makefile
+++ b/Makefile
@@ -488,6 +488,7 @@ OBJCOPY = $(LLVM_PREFIX)llvm-objcopy$(LLVM_SUFFIX)
OBJDUMP = $(LLVM_PREFIX)llvm-objdump$(LLVM_SUFFIX)
READELF = $(LLVM_PREFIX)llvm-readelf$(LLVM_SUFFIX)
STRIP = $(LLVM_PREFIX)llvm-strip$(LLVM_SUFFIX)
+ADDR2LINE = $(LLVM_PREFIX)llvm-addr2line$(LLVM_SUFFIX)
else
CC = $(CROSS_COMPILE)gcc
LD = $(CROSS_COMPILE)ld
@@ -497,6 +498,7 @@ OBJCOPY = $(CROSS_COMPILE)objcopy
OBJDUMP = $(CROSS_COMPILE)objdump
READELF = $(CROSS_COMPILE)readelf
STRIP = $(CROSS_COMPILE)strip
+ADDR2LINE = $(CROSS_COMPILE)addr2line
endif
RUSTC = rustc
RUSTDOC = rustdoc
@@ -611,7 +613,7 @@ export RUSTC_BOOTSTRAP := 1
export ARCH SRCARCH CONFIG_SHELL BASH HOSTCC KBUILD_HOSTCFLAGS CROSS_COMPILE LD CC HOSTPKG_CONFIG
export RUSTC RUSTDOC RUSTFMT RUSTC_OR_CLIPPY_QUIET RUSTC_OR_CLIPPY BINDGEN CARGO
export HOSTRUSTC KBUILD_HOSTRUSTFLAGS
-export CPP AR NM STRIP OBJCOPY OBJDUMP READELF PAHOLE RESOLVE_BTFIDS LEX YACC AWK INSTALLKERNEL
+export CPP AR NM STRIP OBJCOPY OBJDUMP READELF ADDR2LINE PAHOLE RESOLVE_BTFIDS LEX YACC AWK INSTALLKERNEL
export PERL PYTHON3 CHECK CHECKFLAGS MAKE UTS_MACHINE HOSTCXX
export KGZIP KBZIP2 KLZOP LZMA LZ4 XZ ZSTD
export KBUILD_HOSTCXXFLAGS KBUILD_HOSTLDFLAGS KBUILD_HOSTLDLIBS LDFLAGS_MODULE
diff --git a/init/Kconfig b/init/Kconfig
index 6d35728b94b2..d45dd423e1ec 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -1738,6 +1738,28 @@ config KALLSYMS_BASE_RELATIVE
time constants, and no relocation pass is required at runtime to fix
up the entries based on the runtime load address of the kernel.

+config KALLSYMS_ALIAS_SRCLINE
+ bool "Produces alias for duplicated text symbols" if EXPERT
+ depends on KALLSYMS && DEBUG_INFO && !DEBUG_INFO_SPLIT
+ help
+ It is not uncommon for drivers or modules related to similar
+ peripherals to have symbols with the exact same name.
+ While this is not a problem for the kernel's binary itself, it
+ becomes an issue when attempting to trace or probe specific
+ functions using infrastructure like ftrace or kprobe.
+
+ This option addresses this challenge, producing alias for text
+ symbol names that include the file name and line where the symbols
+ are defined in the source code.
+
+config KALLSYMS_ALIAS_SRCLINE_DATA
+ bool "Produces alias also for global variables names"
+ depends on KALLSYMS_ALIAS_SRCLINE
+ help
+ Sometimes it can be useful to refer to global vars by name. Since
+ they suffer the same issue as text symbols, this config option
+ allows having aliases for global variables names too.
+
# end of the "standard kernel features (expert users)" menu

# syscall, maps, verifier
diff --git a/scripts/kas_alias.py b/scripts/kas_alias.py
new file mode 100755
index 000000000000..7c6b7045081c
--- /dev/null
+++ b/scripts/kas_alias.py
@@ -0,0 +1,136 @@
+#!/usr/bin/env python3
+# SPDX-License-Identifier: GPL-2.0-only
+#
+# Copyright (C) 2023 Red Hat, Inc. Alessandro Carminati <alessandro.carminati@gmail.com>
+#
+# kas_alias: Adds alias to duplicate symbols in the kallsyms output.
+
+import subprocess
+import sys
+import os
+import argparse
+import re
+from collections import namedtuple
+
+regex_filter = [
+ "^__compound_literal\\.[0-9]+$",
+ "^__[wm]*key\\.[0-9]+$",
+ "^_*TRACE_SYSTEM.*$",
+ "^__already_done\\.[0-9]+$",
+ "^__msg\\.[0-9]+$",
+ "^__func__\\.[0-9]+$",
+ "^CSWTCH\\.[0-9]+$",
+ "^_rs\\.[0-9]+$",
+ "^___tp_str\\.[0-9]+$",
+ "^__flags\\.[0-9]+$",
+ "^___done\\.[0-9]+$",
+ "^__print_once\\.[0-9]+$",
+ "^___once_key\\.[0-9]+$",
+ "^__pfx_.*$",
+ "^__cfi_.*$"
+ ]
+
+class SeparatorType:
+ def __call__(self, separator):
+ if len(separator) != 1:
+ raise argparse.ArgumentTypeError("Separator must be a single character")
+ return separator
+
+Line = namedtuple('Line', ['address', 'type', 'name'])
+
+def parse_file(filename):
+ symbol_list = []
+ name_occurrences = {}
+
+ with open(filename, 'r') as file:
+ for line in file:
+ fields = line.strip().split()
+
+ if len(fields) >= 3:
+ address, type, name = fields[0], fields[1], ' '.join(fields[2:])
+ symbol_list.append(Line(address, type, name))
+ name_occurrences[name] = name_occurrences.get(name, 0) + 1
+
+ return symbol_list, name_occurrences
+
+def find_duplicate(symbol_list, name_occurrences):
+ name_to_lines = {}
+ duplicate_lines = []
+
+ for line in symbol_list:
+ if line.name in name_to_lines:
+ first_occurrence = name_to_lines[line.name]
+ duplicate_lines.extend([first_occurrence, line])
+ else:
+ name_to_lines[line.name] = line
+
+ return duplicate_lines
+
+def start_addr2line_process(binary_file, addr2line_file):
+ try:
+ addr2line_process = subprocess.Popen([addr2line_file, '-fe', binary_file],
+ stdin=subprocess.PIPE,
+ stdout=subprocess.PIPE,
+ stderr=subprocess.PIPE,
+ text=True)
+ return addr2line_process
+ except Exception as e:
+ print(f"Error starting addr2line process: {str(e)}")
+ sys.exit(1)
+
+def addr2line_fetch_address(addr2line_process, address):
+ try:
+ addr2line_process.stdin.write(address + '\n')
+ addr2line_process.stdin.flush()
+ addr2line_process.stdout.readline().strip()
+ output = addr2line_process.stdout.readline().strip()
+
+ return os.path.normpath(output)
+ except Exception as e:
+ print(f"Error communicating with addr2line: {str(e)}")
+ sys.exit(1)
+
+def process_line(obj, config):
+ if config:
+ return not (any(re.match(regex, obj.name) for regex in regex_filter))
+ else:
+ return obj.type in {"T", "t"}
+
+if __name__ == "__main__":
+ parser = argparse.ArgumentParser(description='Add alias to multiple occurring symbols name in kallsyms')
+ parser.add_argument('-a', "--addr2line", dest="addr2line_file", required=True)
+ parser.add_argument('-v', "--vmlinux", dest="vmlinux_file", required=True)
+ parser.add_argument('-o', "--outfile", dest="output_file", required=True)
+ parser.add_argument('-n', "--nmdata", dest="nm_data_file", required=True)
+ parser.add_argument('-b', "--basedir", dest="linux_base_dir", required=True)
+ parser.add_argument('-s', "--separator", dest="separator", required=False, default="@", type=SeparatorType())
+ parser.add_argument('-d', "--data", dest="include_data", required=False, action='store_true')
+ config = parser.parse_args()
+
+ try:
+ config.linux_base_dir = os.path.normpath(os.getcwd() + "/" + config.linux_base_dir) + "/"
+ symbol_list, name_occurrences = parse_file(config.nm_data_file)
+ addr2line_process = start_addr2line_process(config.vmlinux_file, config.addr2line_file)
+
+ with open(config.output_file, 'w') as file:
+ for obj in symbol_list:
+ file.write(f"{obj.address} {obj.type} {obj.name}\n")
+ if (name_occurrences[obj.name] > 1) and process_line(obj, config.include_data) :
+ output = addr2line_fetch_address(addr2line_process, obj.address)
+ decoration = config.separator + "".join(
+ "_" if not c.isalnum() else c for c in output.replace(config.linux_base_dir, "")
+ )
+ # The addr2line can emit the special string "?:??" when addr2line can not find the
+ # specified address in the DWARF section that after normalization it becomes "____".
+ # In such cases, emitting an alias wouldn't make sense, so it is skipped.
+ if decoration != config.separator + "____":
+ file.write(f"{obj.address} {obj.type} {obj.name + decoration}\n")
+
+ addr2line_process.stdin.close()
+ addr2line_process.stdout.close()
+ addr2line_process.stderr.close()
+ addr2line_process.wait()
+
+ except Exception as e:
+ print(f"An error occurred: {str(e)}")
+ raise SystemExit("Script terminated due to an error")
diff --git a/scripts/link-vmlinux.sh b/scripts/link-vmlinux.sh
index a432b171be82..c110b0f58a19 100755
--- a/scripts/link-vmlinux.sh
+++ b/scripts/link-vmlinux.sh
@@ -91,7 +91,12 @@ vmlinux_link()

# The kallsyms linking does not need debug symbols included.
if [ "$output" != "${output#.tmp_vmlinux.kallsyms}" ] ; then
- ldflags="${ldflags} ${wl}--strip-debug"
+ # The kallsyms linking does not need debug symbols included,
+ # unless the KALLSYMS_ALIAS_SRCLINE.
+ if ! is_enabled CONFIG_KALLSYMS_ALIAS_SRCLINE && \
+ [ "$output" != "${output#.tmp_vmlinux.kallsyms}" ] ; then
+ ldflags="${ldflags} ${wl}--strip-debug"
+ fi
fi

if is_enabled CONFIG_VMLINUX_MAP; then
@@ -161,7 +166,19 @@ kallsyms()
fi

info KSYMS ${2}
- scripts/kallsyms ${kallsymopt} ${1} > ${2}
+ ALIAS=""
+ KAS_DATA=""
+ if is_enabled CONFIG_KALLSYMS_ALIAS_SRCLINE_DATA; then
+ KAS_DATA="--data"
+ fi
+ if is_enabled CONFIG_KALLSYMS_ALIAS_SRCLINE; then
+ ALIAS=".alias"
+ ${srctree}/scripts/kas_alias.py \
+ --addr2line ${ADDR2LINE} --vmlinux ${kallsyms_vmlinux} \
+ --nmdata ${1} --outfile ${1}${ALIAS} \
+ --basedir ${srctree} --separator @ ${KAS_DATA}
+ fi
+ scripts/kallsyms ${kallsymopt} ${1}${ALIAS} > ${2}
}

# Perform one step in kallsyms generation, including temporary linking of
--
2.34.1