Re: [Xen-devel] Ping: [PATCH v2] build: provide option to disambiguate symbol names

[Jan Beulich <jbeulich@xxxxxxxx>]

On 20.11.2019 18:13, Andrew Cooper wrote:
> On 20/11/2019 16:40, Jürgen Groß wrote:
>> On 20.11.19 17:30, Jan Beulich wrote:
>>> On 08.11.2019 12:18, Jan Beulich wrote:
>>>> The .file assembler directives generated by the compiler do not include
>>>> any path components (gcc) or just the ones specified on the command
>>>> line
>>>> (clang, at least version 5), and hence multiple identically named
>>>> source
>>>> files (in different directories) may produce identically named static
>>>> symbols (in their kallsyms representation). The binary diffing
>>>> algorithm
>>>> used by xen-livepatch, however, depends on having unique symbols.
>>>>
>>>> Make the ENFORCE_UNIQUE_SYMBOLS Kconfig option control the (build)
>>>> behavior, and if enabled use objcopy to prepend the (relative to the
>>>> xen/ subdirectory) path to the compiler invoked STT_FILE symbols. Note
>>>> that this build option is made no longer depend on LIVEPATCH, but
>>>> merely
>>>> defaults to its setting now.
>>>>
>>>> Conditionalize explicit .file directive insertion in C files where it
>>>> exists just to disambiguate names in a less generic manner; note that
>>>> at the same time the redundant emission of STT_FILE symbols gets
>>>> suppressed for clang. Assembler files as well as multiply compiled C
>>>> ones using __OBJECT_FILE__ are left alone for the time being.
>>>>
>>>> Since we now expect there not to be any duplicates anymore, also don't
>>>> force the selection of the option to 'n' anymore in allrandom.config.
>>>> Similarly COVERAGE no longer suppresses duplicate symbol warnings if
>>>> enforcement is in effect, which in turn allows
>>>> SUPPRESS_DUPLICATE_SYMBOL_WARNINGS to simply depend on
>>>> !ENFORCE_UNIQUE_SYMBOLS.
>>>>
>>>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>>>
>>> I've got acks from Konrad and Wei, but still need an x86 and a release
>>> one here. Andrew? Or alternatively - Jürgen, would you rather not see
>>> this go in anymore?
>>
>> In case the needed x86 Ack is coming in before RC3 I'm fine to give my
>> Release-ack, but I'm hesitant to take it later.
> 
> Has anyone actually tried building a livepatch with this change in place?

Actually - what is your concern here? The exact spelling of symbols
names should be of no interest to the tool. After all the compiler is
free to invent all sorts of names for its local symbols, including
the ones we would produce with this change in place. All the tool
cares about is that the names be unambiguous. Hence any (theoretical)
regression here would be a bug in the tools, which imo is no reason
to delay this change any further. (Granted I should have got to it
earlier, but it had been continuing to get deferred.)

Jan

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