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

Francis Laniel
2023-10-02T14:27:14Z

Hi.

Le mercredi 27 septembre 2023, 20:35:16 EEST Alessandro Carminati (Red Hat) a
écrit :
> 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@gm
> ail.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@gm
> ail.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@gm
> ail.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

Thank you for sending this new version!
I only found nits and I think we are near its merge!

> https://lore.kernel.org/all/20230919193948.465340-1-alessandro.carminati@gma
> il.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):

This function is never used.

> + 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)

As you already have a try/except block code in your main function, I think it
is better to handle all exception here rather than inside each function.

> +
> +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)

Same comment as above.

> +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.

Thank you for the comment!

> + 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

Best regards.