Re: [PATCH v2 bpf-next 07/13] resolve_btfids: Say which registers an instruction reads

[Anton Protopopov <a.s.protopopov@gmail.com>]

On 26/11/13 05:08AM, Alexei Starovoitov wrote:
> From: Alexei Starovoitov <ast@kernel.org>
> 
> 	Ebusy:
> 		spin_unlock(&files->file_lock);
> 		return -EBUSY;
> 
> 	mov    %rdi,%r13                files, the first argument
> 	...
> 	lea    0xc0(%r13),%rdi          &files->file_lock
> 	call   _raw_spin_unlock
> 	mov    $0xfffffff0,%eax
> 
> do_dup2() unlocks an object of its caller before it returns the error. The
> next patch looks for that in the machine code: is the pointer in %rdi
> computed from an argument of do_dup2()? The decoder says which registers
> an instruction writes. It does not say from what.
> 
> 	text_insn.uses    the registers that the instruction reads: the
> 	                  operands behind the defs of MCInstrDesc, and its
> 	                  implicit uses
> 	text_insn.load    it reads memory: MCInstrDesc::mayLoad()
> 
> 	lea    0xc0(%r13),%rdi          uses r13: a pointer into *files
> 	mov    0xc0(%r13),%rdi          uses r13, load: another object
> 
> An opcode with hasUnmodeledSideEffects() reads all registers, as it writes
> all registers.
> 
> For LLVM "xor %edi,%edi" reads %edi. Compilers pass NULL in this way. The
> table of x86-64 gets a pattern for it: dst = 0.
> 
> A test instruction says what is read only if that is its point: four for
> x86-64, three for arm64.
> 
> No tag changes.
> 
> Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Acked-by: Anton Protopopov <a.s.protopopov@gmail.com>

>  [...]