[CRIU] [PATCH] pie: Give VDSO symbol table local scope

[Christopher Covington]

Hi Laurent,

On 10/05/2015 10:54 AM, Laurent Dufour wrote:
> On 03/10/2015 14:47, Christopher Covington wrote:
>> In commit c2271198, Laurent Dufour kindly reunified the VDSO code
>> that had become duplicated between architectures. Unfortunately
>> this introduced a regression in AArch64 where apparently due to
>> the scope of vdso_symbols array of pointers to characters changing
>> from local to global, load-time relocations became necessary.
>>
>> The following thread on the GCC mailing list discusses why
>> load-time relocations can be necessary when pointers are used,
>> although it doesn't mention the potential for locally scoped
>> arrays to be handled differently:
>> https://gcc.gnu.org/ml/gcc/2004-05/msg01016.html
>>
>> Because the alternatives, such as porting piegen to AArch64, are
>> far more involved, simply revert the change in scope.
> 
> Hi Christopher,
> 
> This breaks the build on powerpc due to the gcc optimisation done on power.
> 
> On power the compiler is calling C library memcpy to copy the
> pre-initialised. This leads to an undefined memcpy symbol when building
> the parasite part.
> I'm looking at the best way to get around that.

Sorry about that. Here are some thoughts:

A) #ifdefs

Ideally with some comments and only temporary until we work out a more
satisfactory solution.

B) Keep the declaration local but make it static const instead of just const

You've probably already seen this, but this approach is mentioned in the
comments here:
http://stackoverflow.com/questions/6410595/getting-gcc-to-compile-without-inserting-call-to-memcpy

C) Pack multiple strings into a single char* that's double-null terminated

Use while() instead of for(), etc.

D) Port piegen to AArch64

I'd like to do this at some point, if only as an exercise to learn more about
ELF and AArch64, but I'm not sure I'll have to the time to do it in the next
few weeks.

Christopher Covington

>> diff --git a/include/util-vdso.h b/include/util-vdso.h
>> index 2942337..c8dfa05 100644
>> --- a/include/util-vdso.h
>> +++ b/include/util-vdso.h
>> @@ -60,10 +60,6 @@ static inline unsigned long vvar_vma_size(struct vdso_symtable *t)
>>  	return t->vvar_end - t->vvar_start;
>>  }
>>
>> -extern const char *vdso_symbols[VDSO_SYMBOL_MAX];
>> -
>>  extern int vdso_fill_symtable(char *mem, size_t size, struct vdso_symtable *t);
>>
>> -
>> -
>>  #endif /* __CR_UTIL_VDSO_H__ */
>> diff --git a/pie/util-vdso.c b/pie/util-vdso.c
>> index 9711aaa..65746d9 100644
>> --- a/pie/util-vdso.c
>> +++ b/pie/util-vdso.c
>> @@ -24,10 +24,6 @@
>>  #endif
>>  #define LOG_PREFIX "vdso: "
>>
>> -const char *vdso_symbols[VDSO_SYMBOL_MAX] = {
>> -	ARCH_VDSO_SYMBOLS
>> -};
>> -
>>  /* Check if pointer is out-of-bound */
>>  static bool __ptr_oob(void *ptr, void *start, size_t size)
>>  {
>> @@ -54,6 +50,10 @@ static unsigned long elf_hash(const unsigned char *name)
>>
>>  int vdso_fill_symtable(char *mem, size_t size, struct vdso_symtable *t)
>>  {
>> +	const char *vdso_symbols[VDSO_SYMBOL_MAX] = {
>> +		ARCH_VDSO_SYMBOLS
>> +	};
>> +
>>  	Elf64_Phdr *dynamic = NULL, *load = NULL;
>>  	Elf64_Ehdr *ehdr = (void *)mem;
>>  	Elf64_Dyn *dyn_strtab = NULL;
>>
> 


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