Commit 6eb4bd92c1ce ("kallsyms: strip LTO suffixes from static functions")
stripped all function/variable suffixes started with '.' regardless
of whether those suffixes are generated at LTO mode or not. In fact,
as far as I know, in LTO mode, when a static function/variable is
promoted to the global scope, '.llvm.<...>' suffix is added.
The existing mechanism breaks live patch for a LTO kernel even if
no <symbol>.llvm.<...> symbols are involved. For example, for the following
kernel symbols:
$ grep bpf_verifier_vlog /proc/kallsyms
ffffffff81549f60 t bpf_verifier_vlog
ffffffff8268b430 d bpf_verifier_vlog._entry
ffffffff8282a958 d bpf_verifier_vlog._entry_ptr
ffffffff82e12a1f d bpf_verifier_vlog.__already_done
'bpf_verifier_vlog' is a static function. '_entry', '_entry_ptr' and
'__already_done' are static variables used inside 'bpf_verifier_vlog',
so llvm promotes them to file-level static with prefix 'bpf_verifier_vlog.'.
Note that the func-level to file-level static function promotion also
happens without LTO.
Given a symbol name 'bpf_verifier_vlog', with LTO kernel, current mechanism will
return 4 symbols to live patch subsystem which current live patching
subsystem cannot handle it. With non-LTO kernel, only one symbol
is returned.
In [1], we have a lengthy discussion, the suggestion is to separate two
cases:
(1). new symbols with suffix which are generated regardless of whether
LTO is enabled or not, and
(2). new symbols with suffix generated only when LTO is enabled.
The cleanup_symbol_name() should only remove suffixes for case (2).
Case (1) should not be changed so it can work uniformly with or without LTO.
This patch removed LTO-only suffix '.llvm.<...>' so live patching and
tracing should work the same way for non-LTO kernel.
[1] https://lore.kernel.org/live-patching/[email protected]/T/#u
Fixes: 6eb4bd92c1ce ("kallsyms: strip LTO suffixes from static functions")
Signed-off-by: Yonghong Song <[email protected]>
---
kernel/kallsyms.c | 5 ++---
1 file changed, 2 insertions(+), 3 deletions(-)
diff --git a/kernel/kallsyms.c b/kernel/kallsyms.c
index 77747391f49b..4874508bb950 100644
--- a/kernel/kallsyms.c
+++ b/kernel/kallsyms.c
@@ -174,11 +174,10 @@ static bool cleanup_symbol_name(char *s)
* LLVM appends various suffixes for local functions and variables that
* must be promoted to global scope as part of LTO. This can break
* hooking of static functions with kprobes. '.' is not a valid
- * character in an identifier in C. Suffixes observed:
+ * character in an identifier in C. Suffixes only in LLVM LTO observed:
* - foo.llvm.[0-9a-f]+
- * - foo.[0-9a-f]+
*/
- res = strchr(s, '.');
+ res = strstr(s, ".llvm.");
if (res) {
*res = '\0';
return true;
--
2.34.1
On 2023/6/28 14:44, Yonghong Song wrote:
> Commit 6eb4bd92c1ce ("kallsyms: strip LTO suffixes from static functions")
> stripped all function/variable suffixes started with '.' regardless
> of whether those suffixes are generated at LTO mode or not. In fact,
> as far as I know, in LTO mode, when a static function/variable is
> promoted to the global scope, '.llvm.<...>' suffix is added.
>
> The existing mechanism breaks live patch for a LTO kernel even if
> no <symbol>.llvm.<...> symbols are involved. For example, for the following
> kernel symbols:
> $ grep bpf_verifier_vlog /proc/kallsyms
> ffffffff81549f60 t bpf_verifier_vlog
> ffffffff8268b430 d bpf_verifier_vlog._entry
> ffffffff8282a958 d bpf_verifier_vlog._entry_ptr
> ffffffff82e12a1f d bpf_verifier_vlog.__already_done
> 'bpf_verifier_vlog' is a static function. '_entry', '_entry_ptr' and
> '__already_done' are static variables used inside 'bpf_verifier_vlog',
> so llvm promotes them to file-level static with prefix 'bpf_verifier_vlog.'.
> Note that the func-level to file-level static function promotion also
> happens without LTO.
>
> Given a symbol name 'bpf_verifier_vlog', with LTO kernel, current mechanism will
> return 4 symbols to live patch subsystem which current live patching
> subsystem cannot handle it. With non-LTO kernel, only one symbol
> is returned.
>
> In [1], we have a lengthy discussion, the suggestion is to separate two
> cases:
> (1). new symbols with suffix which are generated regardless of whether
> LTO is enabled or not, and
> (2). new symbols with suffix generated only when LTO is enabled.
>
> The cleanup_symbol_name() should only remove suffixes for case (2).
> Case (1) should not be changed so it can work uniformly with or without LTO.
>
> This patch removed LTO-only suffix '.llvm.<...>' so live patching and
> tracing should work the same way for non-LTO kernel.
>
> [1] https://lore.kernel.org/live-patching/[email protected]/T/#u
Missed the addition of:
Reported-by: Song Liu <[email protected]>
>
> Fixes: 6eb4bd92c1ce ("kallsyms: strip LTO suffixes from static functions")
> Signed-off-by: Yonghong Song <[email protected]>
> ---
> kernel/kallsyms.c | 5 ++---
> 1 file changed, 2 insertions(+), 3 deletions(-)
>
> diff --git a/kernel/kallsyms.c b/kernel/kallsyms.c
> index 77747391f49b..4874508bb950 100644
> --- a/kernel/kallsyms.c
> +++ b/kernel/kallsyms.c
> @@ -174,11 +174,10 @@ static bool cleanup_symbol_name(char *s)
> * LLVM appends various suffixes for local functions and variables that
> * must be promoted to global scope as part of LTO. This can break
> * hooking of static functions with kprobes. '.' is not a valid
> - * character in an identifier in C. Suffixes observed:
> + * character in an identifier in C. Suffixes only in LLVM LTO observed:
> * - foo.llvm.[0-9a-f]+
> - * - foo.[0-9a-f]+
> */
> - res = strchr(s, '.');
> + res = strstr(s, ".llvm.");
We'd better modify function cleanup_symbol_name() in scripts/kallsyms.c accordingly.
> if (res) {
> *res = '\0';
> return true;
>
--
Regards,
Zhen Lei
On 6/28/23 4:34 AM, Leizhen (ThunderTown) wrote:
>
>
> On 2023/6/28 14:44, Yonghong Song wrote:
>> Commit 6eb4bd92c1ce ("kallsyms: strip LTO suffixes from static functions")
>> stripped all function/variable suffixes started with '.' regardless
>> of whether those suffixes are generated at LTO mode or not. In fact,
>> as far as I know, in LTO mode, when a static function/variable is
>> promoted to the global scope, '.llvm.<...>' suffix is added.
>>
>> The existing mechanism breaks live patch for a LTO kernel even if
>> no <symbol>.llvm.<...> symbols are involved. For example, for the following
>> kernel symbols:
>> $ grep bpf_verifier_vlog /proc/kallsyms
>> ffffffff81549f60 t bpf_verifier_vlog
>> ffffffff8268b430 d bpf_verifier_vlog._entry
>> ffffffff8282a958 d bpf_verifier_vlog._entry_ptr
>> ffffffff82e12a1f d bpf_verifier_vlog.__already_done
>> 'bpf_verifier_vlog' is a static function. '_entry', '_entry_ptr' and
>> '__already_done' are static variables used inside 'bpf_verifier_vlog',
>> so llvm promotes them to file-level static with prefix 'bpf_verifier_vlog.'.
>> Note that the func-level to file-level static function promotion also
>> happens without LTO.
>>
>> Given a symbol name 'bpf_verifier_vlog', with LTO kernel, current mechanism will
>> return 4 symbols to live patch subsystem which current live patching
>> subsystem cannot handle it. With non-LTO kernel, only one symbol
>> is returned.
>>
>> In [1], we have a lengthy discussion, the suggestion is to separate two
>> cases:
>> (1). new symbols with suffix which are generated regardless of whether
>> LTO is enabled or not, and
>> (2). new symbols with suffix generated only when LTO is enabled.
>>
>> The cleanup_symbol_name() should only remove suffixes for case (2).
>> Case (1) should not be changed so it can work uniformly with or without LTO.
>>
>> This patch removed LTO-only suffix '.llvm.<...>' so live patching and
>> tracing should work the same way for non-LTO kernel.
>>
>> [1] https://lore.kernel.org/live-patching/[email protected]/T/#u
>
> Missed the addition of:
> Reported-by: Song Liu <[email protected]>
Will add in the next revision.
>
>>
>> Fixes: 6eb4bd92c1ce ("kallsyms: strip LTO suffixes from static functions")
>> Signed-off-by: Yonghong Song <[email protected]>
>> ---
>> kernel/kallsyms.c | 5 ++---
>> 1 file changed, 2 insertions(+), 3 deletions(-)
>>
>> diff --git a/kernel/kallsyms.c b/kernel/kallsyms.c
>> index 77747391f49b..4874508bb950 100644
>> --- a/kernel/kallsyms.c
>> +++ b/kernel/kallsyms.c
>> @@ -174,11 +174,10 @@ static bool cleanup_symbol_name(char *s)
>> * LLVM appends various suffixes for local functions and variables that
>> * must be promoted to global scope as part of LTO. This can break
>> * hooking of static functions with kprobes. '.' is not a valid
>> - * character in an identifier in C. Suffixes observed:
>> + * character in an identifier in C. Suffixes only in LLVM LTO observed:
>> * - foo.llvm.[0-9a-f]+
>> - * - foo.[0-9a-f]+
>> */
>> - res = strchr(s, '.');
>> + res = strstr(s, ".llvm.");
>
> We'd better modify function cleanup_symbol_name() in scripts/kallsyms.c accordingly.
Thanks, will make corresponding change in scripts/kallsyms.c as well.
>
>
>> if (res) {
>> *res = '\0';
>> return true;
>>
>