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Message-ID: <7d63ddd9.4ade.198a218a61d.Coremail.phoenix500526@163.com>
Date: Wed, 13 Aug 2025 14:23:01 +0800 (CST)
From: 赵佳炜 <phoenix500526@....com>
To: "Yonghong Song" <yonghong.song@...ux.dev>
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:Re: [PATCH v7 2/2] selftests/bpf: Force -O2 for USDT selftests
to cover SIB handling logic
Thanks a lot! I'll file a bug for gcc community.
At 2025-08-13 13:24:39, "Yonghong Song" <yonghong.song@...ux.dev> wrote:
>
>
>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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