c - Can I change the Global Offset Table/GOT or Procedural Linkage Table/PLT programmatically? -


the availability of platform-specific features, such sse or avx, can determined during runtime, useful, if not want compile , ship different objects different features.

the following code example allows me check avx , compiles gcc, provides cpuid.h header:

 #include "stdbool.h"  #include "cpuid.h"   bool has_avx(void)  {      uint32_t eax, ebx, ecx, edx;      __get_cpuid(1, &eax, &ebx, &ecx, &edx);     return ecx & bit_avx;  }

instead of littering code runtime checks, such above, repeatedly perform checks, slow , introduce branching (the checks cached reduce overhead, there branching nonetheless), figured use infrastructure provided dynamic linker/loader.

calls functions external linkage on platforms elf indirect , go through procedural linkage table/plt , global offset table/got.

suppose there 2 internal functions, basic _do_something_basic , somehow optimized version _do_something_avx, uses avx. export generic do_something symbol, , alias basic add: 

static void _do_something_basic(…) {     // basic implementation }   static void _do_something_avx(…) {     // optimized implementation using avx }  void do_something(…) __attribute__((alias("_do_something_basic")));

during load-time of library or program, check availability of avx once using has_avx , depending on result of check point do_something symbol _do_something_avx.

even better be, if point initial version of do_something symbol self-modifying function checks availability of avx using has_avx , replaces _do_something_basic or _do_something_avx.

in theory should possible, how can find location of plt/got programmatically? there abi/api provided elf loader, e.g. ld-linux.so.2, use this? need linker script obtain plt/got location? security considerations, can write plt/got, if obtain pointer it?

maybe project has done or similar already.

i'm aware, solution highly platform-specific, since i'm having deal low-level platform-specific details, features of instruction set, fine.

as others have suggested can go platform-specific versions of libs. or if ok sticking linux, can use (relatively) new ifunc relocations want.

edit: noted sebastian, ifuncs seem supported other platforms (freebsd, android). note however, feature not used may have rough edges.


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