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Message-ID: <CAKwvOdnPWL7ts0PC_NxqxZZRgaT=WLZGQQ-3cVii-jG5GCNjfw@mail.gmail.com>
Date: Tue, 1 Aug 2023 09:42:11 -0700
From: Nick Desaulniers <ndesaulniers@...gle.com>
To: Sami Tolvanen <samitolvanen@...gle.com>
Cc: linux-kernel@...r.kernel.org, kernel-team@...roid.com,
Masahiro Yamada <masahiroy@...nel.org>,
Nathan Chancellor <nathan@...nel.org>,
Nicolas Schier <nicolas@...sle.eu>,
Josh Poimboeuf <jpoimboe@...nel.org>,
John Stultz <jstultz@...gle.com>, linux-kbuild@...r.kernel.org,
Will Deacon <will@...nel.org>,
Fangrui Song <maskray@...gle.com>
Subject: Re: [PATCH v3 3/4] scripts/faddr2line: Constrain readelf output to
symbols from System.map
On Fri, Jul 28, 2023 at 4:34 AM Will Deacon <will@...nel.org> wrote:
>
> Some symbols emitted in the readelf output but filtered from System.map
> can confuse the 'faddr2line' symbol size calculation, resulting in the
> erroneous rejection of valid offsets. This is especially prevalent when
> building an arm64 kernel with CONFIG_CFI_CLANG=y, where most functions
> are prefixed with a 32-bit data value in a '$d.n' section. For example:
>
> 447538: ffff800080014b80 548 FUNC GLOBAL DEFAULT 2 do_one_initcall
> 104: ffff800080014c74 0 NOTYPE LOCAL DEFAULT 2 $x.73
> 106: ffff800080014d30 0 NOTYPE LOCAL DEFAULT 2 $x.75
> 111: ffff800080014da4 0 NOTYPE LOCAL DEFAULT 2 $d.78
> 112: ffff800080014da8 0 NOTYPE LOCAL DEFAULT 2 $x.79
> 36: ffff800080014de0 200 FUNC LOCAL DEFAULT 2 run_init_process
Sami,
Should we change the llvm-ir linkage type for these symbols from
`internal` to `private`?
https://llvm.org/docs/LangRef.html#linkage-types
Then they would not appear in the symbol table.
At first, I thought other modules might need to directly reference
this data, but with the local binding, I don't think they can.
>
> Adding a warning to do_one_initcall() results in:
>
> | WARNING: CPU: 0 PID: 1 at init/main.c:1236 do_one_initcall+0xf4/0x260
>
> Which 'faddr2line' refuses to accept:
>
> $ ./scripts/faddr2line vmlinux do_one_initcall+0xf4/0x260
> skipping do_one_initcall address at 0xffff800080014c74 due to size mismatch (0x260 != 0x224)
> no match for do_one_initcall+0xf4/0x260
>
> Filter out entries from readelf using the 'sysmap-ignored-syms.sed'
> script used to construct System.map, so that the size of a symbol is
> calculated as a delta to the next symbol present in ksymtab.
>
> Cc: Josh Poimboeuf <jpoimboe@...nel.org>
> Cc: John Stultz <jstultz@...gle.com>
> Signed-off-by: Will Deacon <will@...nel.org>
> ---
> scripts/faddr2line | 3 ++-
> 1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/scripts/faddr2line b/scripts/faddr2line
> index 62a3fa6f6f59..da734af90036 100755
> --- a/scripts/faddr2line
> +++ b/scripts/faddr2line
> @@ -64,6 +64,7 @@ else
> UTIL_PREFIX=${CROSS_COMPILE:-}
> fi
>
> +IGNORED_SYMS=$(dirname $0)/sysmap-ignored-syms.sed
> READELF="${UTIL_PREFIX}readelf"
> ADDR2LINE="${UTIL_PREFIX}addr2line"
> AWK="awk"
> @@ -185,7 +186,7 @@ __faddr2line() {
> found=2
> break
> fi
> - done < <(${READELF} --symbols --wide $objfile | sed 's/\[.*\]//' | ${AWK} -v sec=$sym_sec '$7 == sec' | sort --key=2)
> + done < <(${READELF} --symbols --wide $objfile | sed -f ${IGNORED_SYMS} -e 's/\[.*\]//' | ${AWK} -v sec=$sym_sec '$7 == sec' | sort --key=2)
>
> if [[ $found = 0 ]]; then
> warn "can't find symbol: sym_name: $sym_name sym_sec: $sym_sec sym_addr: $sym_addr sym_elf_size: $sym_elf_size"
> --
> 2.41.0.487.g6d72f3e995-goog
>
--
Thanks,
~Nick Desaulniers
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