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Message-ID: <533437a4-a76d-96e0-b04a-ab8eb7b5fb7f@meta.com>
Date: Mon, 1 May 2023 08:23:35 -0700
From: Yonghong Song <yhs@...a.com>
To: Espen Grindhaug <espen.grindhaug@...il.com>
Cc: Andrii Nakryiko <andrii@...nel.org>,
Alexei Starovoitov <ast@...nel.org>,
Daniel Borkmann <daniel@...earbox.net>,
Martin KaFai Lau <martin.lau@...ux.dev>,
Song Liu <song@...nel.org>, Yonghong Song <yhs@...com>,
John Fastabend <john.fastabend@...il.com>,
KP Singh <kpsingh@...nel.org>,
Stanislav Fomichev <sdf@...gle.com>,
Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
Mykola Lysenko <mykolal@...com>, Shuah Khan <shuah@...nel.org>,
bpf@...r.kernel.org, linux-kernel@...r.kernel.org,
linux-kselftest@...r.kernel.org
Subject: Re: [PATCH v2] libbpf: Improve version handling when attaching uprobe
On 5/1/23 6:00 AM, Espen Grindhaug wrote:
> On Thu, Apr 27, 2023 at 06:19:29PM -0700, Yonghong Song wrote:
>>
>>
>> On 4/27/23 12:19 PM, Espen Grindhaug wrote:
>>> On Wed, Apr 26, 2023 at 02:47:27PM -0700, Yonghong Song wrote:
>>>>
>>>>
>>>> On 4/23/23 11:55 AM, Espen Grindhaug wrote:
>>>>> This change fixes the handling of versions in elf_find_func_offset.
>>>>> In the previous implementation, we incorrectly assumed that the
>>>>
>>>> Could you give more explanation/example in the commit message
>>>> what does 'incorrectly' mean here? In which situations the
>>>> current libbpf implementation will not be correct?
>>>>
>>>
>>> How about something like this?
>>>
>>>
>>> libbpf: Improve version handling when attaching uprobe
>>>
>>> This change fixes the handling of versions in elf_find_func_offset.
>>>
>>> For example, let's assume we are trying to attach an uprobe to pthread_create in
>>> glibc. Prior to this commit, it would fail with an error message saying 'elf:
>>> ambiguous match [...]', this is because there are two entries in the symbol
>>> table with that name.
>>>
>>> $ nm -D /lib/x86_64-linux-gnu/libc.so.6 | grep pthread_create
>>> 0000000000094cc0 T pthread_create@...BC_2.2.5
>>> 0000000000094cc0 T pthread_create@@GLIBC_2.34
>>>
>>> So we go ahead and modify our code to attach to 'pthread_create@@GLIBC_2.34',
>>> and this also fails, but this time with the error 'elf: failed to find symbol
>>> [...]'. This fails because we incorrectly assumed that the version information
>>> would be present in the string found in the string table, but there is only the
>>> string 'pthread_create'.
>>
>> I tried one example with my centos8 libpthread library.
>>
>> $ llvm-readelf -s /lib64/libc-2.28.so | grep pthread_cond_signal
>> 39: 0000000000095f70 43 FUNC GLOBAL DEFAULT 14
>> pthread_cond_signal@@GLIBC_2.3.2
>> 40: 0000000000096250 43 FUNC GLOBAL DEFAULT 14
>> pthread_cond_signal@...BC_2.2.5
>> 3160: 0000000000096250 43 FUNC LOCAL DEFAULT 14
>> __pthread_cond_signal_2_0
>> 3589: 0000000000095f70 43 FUNC LOCAL DEFAULT 14
>> __pthread_cond_signal
>> 5522: 0000000000095f70 43 FUNC GLOBAL DEFAULT 14
>> pthread_cond_signal@@GLIBC_2.3.2
>> 5545: 0000000000096250 43 FUNC GLOBAL DEFAULT 14
>> pthread_cond_signal@...BC_2.2.5
>> $ nm -D /lib64/libc-2.28.so | grep pthread_cond_signal
>> 0000000000095f70 T pthread_cond_signal@@GLIBC_2.3.2
>> 0000000000096250 T pthread_cond_signal@...BC_2.2.5
>> $
>>
>> Note that two pthread_cond_signal functions have different addresses,
>> which is expected as they implemented for different versions.
>>
>> But in your case,
>>> $ nm -D /lib/x86_64-linux-gnu/libc.so.6 | grep pthread_create
>>> 0000000000094cc0 T pthread_create@...BC_2.2.5
>>> 0000000000094cc0 T pthread_create@@GLIBC_2.34
>>
>> Two functions have the same address which is very weird and I suspect
>> some issues here at least needs some investigation.
>>
>
> I am no expert on this, but as far as I can tell, this is normal,
> although much more common on my Ubuntu machine than my Fedora machine.
>
> Script to find duplicates:
>
> nm -D /usr/lib64/libc-2.33.so | awk '
> {
> addr = $1;
> symbol = $3;
> sub(/[@].*$/, "", symbol);
>
> if (addr == prev_addr && symbol == prev_symbol) {
> if (prev_symbol_printed == 0) {
> print prev_line;
> prev_symbol_printed = 1;
> }
> print;
> } else {
> prev_symbol_printed = 0;
> }
> prev_addr = addr;
> prev_symbol = symbol;
> prev_line = $0;
> }'
>
>
>> Second, for the symbol table, the following is ELF encoding,
>>
>> typedef struct {
>> Elf64_Word st_name;
>> unsigned char st_info;
>> unsigned char st_other;
>> Elf64_Half st_shndx;
>> Elf64_Addr st_value;
>> Elf64_Xword st_size;
>> } Elf64_Sym;
>>
>> where
>> st_name
>>
>> An index into the object file's symbol string table, which holds the
>> character representations of the symbol names. If the value is nonzero, the
>> value represents a string table index that gives the symbol name. Otherwise,
>> the symbol table entry has no name.
>>
>> So, the function name (including @..., @@...) should be in string table
>> which is the same for the above two pthread_cond_signal symbols.
>>
>> I think it is worthwhile to debug why in your situation
>> pthread_create@...BC_2.2.5 and pthread_create@@GLIBC_2.34 do not
>> have them in the string table.
>>
>
> I think you are mistaken here; the strings in the strings table don't contain
> the version. Take a look at this partial dump of the strings table.
>
> $ readelf -W -p .dynstr /usr/lib64/libc-2.33.so
>
> String dump of section '.dynstr':
> [ 1] xdrmem_create
> [ f] __wctomb_chk
> [ 1c] getmntent
> [ 26] __freelocale
> [ 33] __rawmemchr
> [ 3f] _IO_vsprintf
> [ 4c] getutent
> [ 55] __file_change_detection_for_path
> (...)
> [ 350e] memrchr
> [ 3516] pthread_cond_signal
> [ 352a] __close
> (...)
> [ 61b6] GLIBC_2.2.5
> [ 61c2] GLIBC_2.2.6
> [ 61ce] GLIBC_2.3
> [ 61d8] GLIBC_2.3.2
> [ 61e4] GLIBC_2.3.3
>
> As you can see, the strings have no versions, and the version strings
> themselves are also in this table as entries at the end of the table.
I see you search .dynstr section. Do you think whether we should
search .strtab instead since it contains versioned symbols?
>
>>>
>>> This patch reworks how we compare the symbol name provided by the user if it is
>>> qualified with a version (using @ or @@). We now look up the correct version
>>> string in the version symbol table before constructing the full name, as also
>>> done above by nm, before comparing.
>>>
>>>>> version information would be present in the string found in the
>>>>> string table.
>>>>>
>>>>> We now look up the correct version string in the version symbol
>>>>> table before constructing the full name and then comparing.
>>>>>
>>>>> This patch adds support for both name@...sion and name@@version to
>>>>> match output of the various elf parsers.
>>>>>
>>>>> Signed-off-by: Espen Grindhaug <espen.grindhaug@...il.com>
>>>>
>>>> [...]
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