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Message-ID: <6c444d7d-524d-4bc8-bda6-0440af621ebe@linux.dev>
Date: Tue, 12 Aug 2025 22:24:39 -0700
From: Yonghong Song <yonghong.song@...ux.dev>
To: 赵佳炜 <phoenix500526@....com>
Cc: ast@...nel.org, daniel@...earbox.net, andrii@...nel.org,
bpf@...r.kernel.org, linux-kselftest@...r.kernel.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH v7 2/2] selftests/bpf: Force -O2 for USDT selftests to
cover SIB handling logic
On 8/12/25 7:27 PM, 赵佳炜 wrote:
>
>
>
>
>
> Sure.
>
> The usdt_rip.c source code:
>
> ```C
> // the usdt_rip.c file
> #include <stddef.h>
> #include <stdint.h>
> #include "sdt.h"
>
> static volatile char ti = 0;
>
> int add(int a, int b) {
> return a + b;
> }
> int (*add_ptr)(int, int) = add;
>
> struct st{
> int a;
> char b;
> };
>
> volatile struct st t1 = {.a = 1, .b = 'a'};
>
> static void __always_inline trigger_func() {
> STAP_PROBE4(usdt_rip, rip_global_var, ti, add_ptr, t1.b, ti);
> }
>
> int main() {
> trigger_func();
> return 0;
> }
> ```
>
> The test/usdt_rip.c source code:
>
> ```C
> // The test/usdt_rip.c file
> static volatile char ti = 0;
> ```
>
> The compiler option is `gcc -O2 -g usdt_rip.c test/usdt_rip.c -o usdt_rip`
>
> My compiler version:
> ```bash
> $ gcc -v
> Using built-in specs.
> COLLECT_GCC=gcc
> COLLECT_LTO_WRAPPER=/usr/libexec/gcc/x86_64-linux-gnu/13/lto-wrapper
> OFFLOAD_TARGET_NAMES=nvptx-none:amdgcn-amdhsa
> OFFLOAD_TARGET_DEFAULT=1
> Target: x86_64-linux-gnu
> Configured with: ../src/configure -v --with-pkgversion='Ubuntu 13.3.0-6ubuntu2~24.04' --with-bugurl=file:///usr/share/doc/gcc-13/README.Bugs --enable-languages=c,ada,c++,go,d,fortran,objc,obj-c++,m2 --prefix=/usr --with-gcc-major-version-only --program-suffix=-13 --program-prefix=x86_64-linux-gnu- --enable-shared --enable-linker-build-id --libexecdir=/usr/libexec --without-included-gettext --enable-threads=posix --libdir=/usr/lib --enable-nls --enable-bootstrap --enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes --with-default-libstdcxx-abi=new --enable-libstdcxx-backtrace --enable-gnu-unique-object --disable-vtable-verify --enable-plugin --enable-default-pie --with-system-zlib --enable-libphobos-checking=release --with-target-system-zlib=auto --enable-objc-gc=auto --enable-multiarch --disable-werror --enable-cet --with-arch-32=i686 --with-abi=m64 --with-multilib-list=m32,m64,mx32 --enable-multilib --with-tune=generic --enable-offload-targets=nvptx-none=/build/gcc-13-fG75Ri/gcc-13-13.3.0/debian/tmp-nvptx/usr,amdgcn-amdhsa=/build/gcc-13-fG75Ri/gcc-13-13.3.0/debian/tmp-gcn/usr --enable-offload-defaulted --without-cuda-driver --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu --with-build-config=bootstrap-lto-lean --enable-link-serialization=2
> Thread model: posix
> Supported LTO compression algorithms: zlib zstd
> gcc version 13.3.0 (Ubuntu 13.3.0-6ubuntu2~24.04)
> ```
Thanks for the code. I compiled with gcc14 compiler and the below is the asm code for 'main' function:
0000000000401020 <main>:
401020: 90 nop
401021: 31 c0 xor %eax,%eax
401023: c3 ret
401024: 66 2e 0f 1f 84 00 00 cs nopw 0x0(%rax,%rax,1)
40102b: 00 00 00
40102e: 66 90 xchg %ax,%ax
stapsdt note:
Displaying notes found in: .note.stapsdt
Owner Data size Description
stapsdt 0x00000066 NT_STAPSDT (SystemTap probe descriptors)
Provider: usdt_rip
Name: rip_global_var
Location: 0x0000000000401020, Base: 0x0000000000402010, Semaphore: 0x0000000000000000
Arguments: -1@ti(%rip) 8@..._ptr(%rip) -1@...4(%rip) -1@ti(%rip)
$ readelf -s usdt_rip | grep ti
4: 0000000000404019 1 OBJECT LOCAL DEFAULT 24 ti
14: 000000000040401a 1 OBJECT LOCAL DEFAULT 24 ti
So yet, it would be hard to find which 'ti' should be used to resolve usdt arguments.
I then tried with clang21. The 'main' function:
0000000000001140 <main>:
1140: 0f b6 05 f2 2e 00 00 movzbl 0x2ef2(%rip),%eax # 4039 <ti>
1147: 48 8b 0d da 2e 00 00 mov 0x2eda(%rip),%rcx # 4028 <add_ptr>
114e: 0f b6 15 df 2e 00 00 movzbl 0x2edf(%rip),%edx # 4034 <t1+0x4>
1155: 0f b6 35 dd 2e 00 00 movzbl 0x2edd(%rip),%esi # 4039 <ti>
115c: 88 44 24 f7 mov %al,-0x9(%rsp)
1160: 48 89 4c 24 f8 mov %rcx,-0x8(%rsp)
1165: 88 54 24 f6 mov %dl,-0xa(%rsp)
1169: 40 88 74 24 f5 mov %sil,-0xb(%rsp)
116e: 90 nop
116f: 31 c0 xor %eax,%eax
1171: c3 ret
1172: 66 90 xchg %ax,%ax
For this case, usdt should work properly. The following is related symbol table:
74: 0000000000004028 8 OBJECT GLOBAL DEFAULT 26 add_ptr
64: 0000000000004030 8 OBJECT GLOBAL DEFAULT 26 t1
50: 0000000000004039 1 OBJECT LOCAL DEFAULT 27 ti <=== for '# 4039 <ti>'.
And for llvm21, only one 'ti' in the symbol table.
$ readelf -s usdt_rip | grep ti
50: 0000000000004039 1 OBJECT LOCAL DEFAULT 27 ti
The stapsdt note:
Displaying notes found in: .note.stapsdt
Owner Data size Description
stapsdt 0x00000061 NT_STAPSDT (SystemTap probe descriptors)
Provider: usdt_rip
Name: rip_global_var
Location: 0x000000000000116e, Base: 0x0000000000002004, Semaphore: 0x0000000000000000
Arguments: -1@-9(%rsp) 8@-8(%rsp) -1@-10(%rsp) -1@-11(%rsp)
It looks like clang optimized away the following:
// The test/usdt_rip.c file
static volatile char ti = 0;
but gcc does not. But even if the above 'ti' is preserved for clang,
clang generated code should still be fine since the argument
is -1@-9(%rsp) 8@-8(%rsp) -1@-10(%rsp) -1@-11(%rsp).
I think you can file a bug for gcc community.
>
>
> At 2025-08-13 00:11:45, "Yonghong Song" <yonghong.song@...ux.dev> wrote:
>>
>> On 8/12/25 12:02 AM, 赵佳炜 wrote:
>>> Yes, I've tried that but it didn't help. FYI:
>>>
>>> $ readelf -nsr usdt_rip
>>>
>>>
>>> Relocation section '.rela.dyn' at offset 0x530 contains 9 entries:
>>> Offset Info Type Sym. Value Sym. Name + Addend
>>> 000000003df0 000000000008 R_X86_64_RELATIVE 1150
>>> 000000003df8 000000000008 R_X86_64_RELATIVE 1110
>>> 000000004008 000000000008 R_X86_64_RELATIVE 4008
>>> 000000004018 000000000008 R_X86_64_RELATIVE 1160
>>> 000000003fd8 000100000006 R_X86_64_GLOB_DAT 0000000000000000 __libc_start_main@...BC_2.34 + 0
>>> 000000003fe0 000200000006 R_X86_64_GLOB_DAT 0000000000000000 _ITM_deregisterTM[...] + 0
>>> 000000003fe8 000300000006 R_X86_64_GLOB_DAT 0000000000000000 __gmon_start__ + 0
>>> 000000003ff0 000400000006 R_X86_64_GLOB_DAT 0000000000000000 _ITM_registerTMCl[...] + 0
>>> 000000003ff8 000500000006 R_X86_64_GLOB_DAT 0000000000000000 __cxa_finalize@...BC_2.2.5 + 0
>>>
>>>
>>> Symbol table '.dynsym' contains 6 entries:
>>> Num: Value Size Type Bind Vis Ndx Name
>>> 0: 0000000000000000 0 NOTYPE LOCAL DEFAULT UND
>>> 1: 0000000000000000 0 FUNC GLOBAL DEFAULT UND _[...]@GLIBC_2.34 (2)
>>> 2: 0000000000000000 0 NOTYPE WEAK DEFAULT UND _ITM_deregisterT[...]
>>> 3: 0000000000000000 0 NOTYPE WEAK DEFAULT UND __gmon_start__
>>> 4: 0000000000000000 0 NOTYPE WEAK DEFAULT UND _ITM_registerTMC[...]
>>> 5: 0000000000000000 0 FUNC WEAK DEFAULT UND [...]@GLIBC_2.2.5 (3)
>>>
>>>
>>> Symbol table '.symtab' contains 42 entries:
>>> Num: Value Size Type Bind Vis Ndx Name
>>> 0: 0000000000000000 0 NOTYPE LOCAL DEFAULT UND
>>> 1: 0000000000000000 0 FILE LOCAL DEFAULT ABS Scrt1.o
>>> 2: 000000000000038c 32 OBJECT LOCAL DEFAULT 4 __abi_tag
>>> 3: 0000000000000000 0 FILE LOCAL DEFAULT ABS usdt_rip.c
>>> 4: 0000000000004021 1 OBJECT LOCAL DEFAULT 25 ti
>>> 5: 0000000000000000 0 FILE LOCAL DEFAULT ABS crtstuff.c
>>> 6: 00000000000010a0 0 FUNC LOCAL DEFAULT 14 deregister_tm_clones
>>> 7: 00000000000010d0 0 FUNC LOCAL DEFAULT 14 register_tm_clones
>>> 8: 0000000000001110 0 FUNC LOCAL DEFAULT 14 __do_global_dtors_aux
>>> 9: 0000000000004020 1 OBJECT LOCAL DEFAULT 25 completed.0
>>> 10: 0000000000003df8 0 OBJECT LOCAL DEFAULT 21 __do_global_dtor[...]
>>> 11: 0000000000001150 0 FUNC LOCAL DEFAULT 14 frame_dummy
>>> 12: 0000000000003df0 0 OBJECT LOCAL DEFAULT 20 __frame_dummy_in[...]
>>> 13: 0000000000000000 0 FILE LOCAL DEFAULT ABS damo.c
>>> 14: 0000000000004022 1 OBJECT LOCAL DEFAULT 25 ti
>>> 15: 0000000000000000 0 FILE LOCAL DEFAULT ABS crtstuff.c
>>> 16: 00000000000020d8 0 OBJECT LOCAL DEFAULT 19 __FRAME_END__
>>> 17: 0000000000000000 0 FILE LOCAL DEFAULT ABS
>>> 18: 0000000000003e00 0 OBJECT LOCAL DEFAULT 22 _DYNAMIC
>>> 19: 0000000000002008 0 NOTYPE LOCAL DEFAULT 18 __GNU_EH_FRAME_HDR
>>> 20: 0000000000003fc0 0 OBJECT LOCAL DEFAULT 23 _GLOBAL_OFFSET_TABLE_
>>> 21: 0000000000000000 0 FUNC GLOBAL DEFAULT UND __libc_start_mai[...]
>>> 22: 0000000000000000 0 NOTYPE WEAK DEFAULT UND _ITM_deregisterT[...]
>>> 23: 0000000000004000 0 NOTYPE WEAK DEFAULT 24 data_start
>>> 24: 0000000000001160 8 FUNC GLOBAL DEFAULT 14 add
>>> 25: 0000000000004020 0 NOTYPE GLOBAL DEFAULT 24 _edata
>>> 26: 0000000000002004 1 NOTYPE WEAK HIDDEN 17 _.stapsdt.base
>>> 27: 0000000000004010 8 OBJECT GLOBAL DEFAULT 24 t1
>>> 28: 0000000000001168 0 FUNC GLOBAL HIDDEN 15 _fini
>>> 29: 0000000000004000 0 NOTYPE GLOBAL DEFAULT 24 __data_start
>>> 30: 0000000000000000 0 NOTYPE WEAK DEFAULT UND __gmon_start__
>>> 31: 0000000000004008 0 OBJECT GLOBAL HIDDEN 24 __dso_handle
>>> 32: 0000000000002000 4 OBJECT GLOBAL DEFAULT 16 _IO_stdin_used
>>> 33: 0000000000004028 0 NOTYPE GLOBAL DEFAULT 25 _end
>>> 34: 0000000000001070 38 FUNC GLOBAL DEFAULT 14 _start
>>> 35: 0000000000004020 0 NOTYPE GLOBAL DEFAULT 25 __bss_start
>>> 36: 0000000000001040 48 FUNC GLOBAL DEFAULT 14 main
>>> 37: 0000000000004018 8 OBJECT GLOBAL DEFAULT 24 add_ptr
>>> 38: 0000000000004020 0 OBJECT GLOBAL HIDDEN 24 __TMC_END__
>>> 39: 0000000000000000 0 NOTYPE WEAK DEFAULT UND _ITM_registerTMC[...]
>>> 40: 0000000000000000 0 FUNC WEAK DEFAULT UND __cxa_finalize@G[...]
>>> 41: 0000000000001000 0 FUNC GLOBAL HIDDEN 11 _init
>>>
>>>
>>> Displaying notes found in: .note.gnu.property
>>> Owner Data size Description
>>> GNU 0x00000020 NT_GNU_PROPERTY_TYPE_0
>>> Properties: x86 feature: IBT, SHSTK
>>> x86 ISA needed: x86-64-baseline
>>>
>>>
>>> Displaying notes found in: .note.gnu.build-id
>>> Owner Data size Description
>>> GNU 0x00000014 NT_GNU_BUILD_ID (unique build ID bitstring)
>>> Build ID: eb615daa575687cc44edc1d339b27890c12c27f1
>>>
>>>
>>> Displaying notes found in: .note.ABI-tag
>>> Owner Data size Description
>>> GNU 0x00000010 NT_GNU_ABI_TAG (ABI version tag)
>>> OS: Linux, ABI: 3.2.0
>>>
>>>
>>> Displaying notes found in: .note.stapsdt
>>> Owner Data size Description
>>> stapsdt 0x00000066 NT_STAPSDT (SystemTap probe descriptors)
>>> Provider: usdt_rip
>>> Name: rip_global_var
>>> Location: 0x0000000000001058, Base: 0x0000000000002004, Semaphore: 0x0000000000000000
>>> Arguments: -1@ti(%rip) 8@..._ptr(%rip) -1@...1(%rip) -1@ti(%rip)
>> Could you share the complete source codes and compiler options which
>> reproduce the above result?
>>
>>>
>>>
>>> At 2025-08-12 13:06:40, "Yonghong Song" <yonghong.song@...ux.dev> wrote:
>>>> On 8/10/25 1:55 AM, 赵佳炜 wrote:
>>>>> Hi Yonghong,
>>>>>
>>>>> I found another issue where symbols can be duplicated, and I’m not sure how to tell them apart.
>>>>>
>>>>> For example, I created two C files named usdt_rip.c and hello.c. Both define their own static ti variables, like:`static volatile char ti = 0;`.
>>>>>
>>>>> After compiling, I obtained an ELF file usdt_rip whose .symtab contains the following entries:
>>>>>
>>>>> $ readelf -s usdt_rip
>>>>>
>>>>> Symbol table '.symtab' contains 42 entries:
>>>>> Num: Value Size Type Bind Vis Ndx Name
>>>>> 0: 0000000000000000 0 NOTYPE LOCAL DEFAULT UND
>>>>> 1: 0000000000000000 0 FILE LOCAL DEFAULT ABS Scrt1.o
>>>>> 2: 000000000000038c 32 OBJECT LOCAL DEFAULT 4 __abi_tag
>>>>> 3: 0000000000000000 0 FILE LOCAL DEFAULT ABS usdt_rip.c
>>>>> 4: 0000000000004021 1 OBJECT LOCAL DEFAULT 25 ti
>>>>> 5: 0000000000000000 0 FILE LOCAL DEFAULT ABS crtstuff.c
>>>>> 6: 00000000000010a0 0 FUNC LOCAL DEFAULT 14 deregister_tm_clones
>>>>> 7: 00000000000010d0 0 FUNC LOCAL DEFAULT 14 register_tm_clones
>>>>> 8: 0000000000001110 0 FUNC LOCAL DEFAULT 14 __do_global_dtors_aux
>>>>> 9: 0000000000004020 1 OBJECT LOCAL DEFAULT 25 completed.0
>>>>> 10: 0000000000003df8 0 OBJECT LOCAL DEFAULT 21 __do_global_dtor[...]
>>>>> 11: 0000000000001150 0 FUNC LOCAL DEFAULT 14 frame_dummy
>>>>> 12: 0000000000003df0 0 OBJECT LOCAL DEFAULT 20 __frame_dummy_in[...]
>>>>> 13: 0000000000000000 0 FILE LOCAL DEFAULT ABS damo.c
>>>>> 14: 0000000000004022 1 OBJECT LOCAL DEFAULT 25 ti
>>>>> 15: 0000000000000000 0 FILE LOCAL DEFAULT ABS crtstuff.c
>>>>> 16: 00000000000020d8 0 OBJECT LOCAL DEFAULT 19 __FRAME_END__
>>>>>
>>>>>
>>>>> As you can see, there are two ti variables in the .symtab section. Their values are very close, making them hard to distinguish.
>>>>>
>>>>> I’m unsure how to handle this situation. Do you have any suggestions?
>>>> Did you check relocations? Relocaitons should be able to point exact which symbol.
>>>>
>>>>> Thanks,
>>>>> Jiawei Zhao
>>>> [...]
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