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Message-ID: <671c0c16.82cb.198c1bcc212.Coremail.phoenix500526@163.com>
Date: Tue, 19 Aug 2025 17:50:29 +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
> libbpf/usdt can reject that if 'ti' is a static variable.
But how can we do that? It seems we cannot determine whether a variable is static
during the preprocessing phase.
At 2025-08-19 01:35:04, "Yonghong Song" <yonghong.song@...ux.dev> wrote:
>
>
>On 8/17/25 6:43 AM, 赵佳炜 wrote:
>>
>>
>>
>>
>>
>>
>> Hi, Yonghong. I've already filed an issue[1] in GCC community.
>>
>>
>> Accroding to the discussion, it's not a gcc bug but may be a systemtap bug.
>> I don't know how to report this bug to systemtap, but I found that the
>> libbpf/usdt have the same problem. I've filed an issue in libbpf/usdt repo[2].
>>
>> I also have some ideas about it. I wrote it down in the issue[2] comment.
>> May be we can discuss there.
>>
>> [1]. https://gcc.gnu.org/bugzilla/show_bug.cgi?id=121569
>> [2]. https://github.com/libbpf/usdt/issues/13
>
>Thanks for filing an issue on gcc and getting some feedback/suggestions
>from gcc community.
>
>Currently, libbpf/usdt does not suport format like '-1@ti(%rip)'. If we do
>intend to implement this. libbpf/usdt can reject that if 'ti' is a
>static variable. libbpf can provide some hints about how to make it
>work (see above [1] and [2]). Then, it would be user's reponsibility to
>change code so libbpf can support it.
>
>>
>>
>>
>> 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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