What does this mean in Valgrind's VALGRIND_DO_CLIENT_REQUEST_EXPR?
__asm__ volatile (
__SPECIAL_INSTRUCTION_PREAMBLE
/* %RDX = client_request ( %RAX ) */
"xchgq %rbx, %rbx"
: "=d" (_zzq_result)
: "a" (&_zzq_args[0]), "0" (_zzq_default)
: "cc", "memory"
)
Where __SPECIAL_INSTRUCTION_PREAMBLE is defined as:
#define __SPECIAL_INSTRUCTION_PREAMBLE \
"rolq $3, %%rdi ; rolq $13, %%rdi\n\t" \
"rolq $61, %%rdi ; rolq $51, %%rdi\n\t"
Seems like some black magic.
There is no way for a 'client' executable to interact with the Valgrind host directly. The client and host have separate memory spaces. Unlike tools such as Purify and the Sanitizers Valgrind can't provide any C functions for clients to be able to introspect any information from the Valgrind host.
The client request mechanism uses a short asm preamble as a marker for the Valgrind host. This is a sequence of instructions that has no side effects and is also a sequence that no compiler will ever emit. It would be a big problem if ever compilers generated code that Valgrind incorrrectly saw as a client request. The amd64 example that you gave uses 4 left rotates of RDI with a total of 128 bits, leaving it unchanged. That's followed by an exchange of RBX with itself. Other CPU architectures use similar 'no-op' sequences to signal client requests.
Since these instructions do nothing they can be compiled into release builds with only a very small size and runtime overhead.
Here is the comment in the Valgrind source that explains these special instructions (in VEX/priv/guest_amd64_toIR.c - in Valgrind we use 'guest' and 'client' interchangeably).
/* "Special" instructions.
This instruction decoder can decode three special instructions
which mean nothing natively (are no-ops as far as regs/mem are
concerned) but have meaning for supporting Valgrind. A special
instruction is flagged by the 16-byte preamble 48C1C703 48C1C70D
48C1C73D 48C1C733 (in the standard interpretation, that means: rolq
$3, %rdi; rolq $13, %rdi; rolq $61, %rdi; rolq $51, %rdi).
Following that, one of the following 3 are allowed (standard
interpretation in parentheses):
4887DB (xchgq %rbx,%rbx) %RDX = client_request ( %RAX )
4887C9 (xchgq %rcx,%rcx) %RAX = guest_NRADDR
4887D2 (xchgq %rdx,%rdx) call-noredir *%RAX
4887F6 (xchgq %rdi,%rdi) IR injection
Any other bytes following the 16-byte preamble are illegal and
constitute a failure in instruction decoding. This all assumes
that the preamble will never occur except in specific code
fragments designed for Valgrind to catch.
No prefixes may precede a "Special" instruction.
*/
and the corrrespongding code
/* Spot "Special" instructions (see comment at top of file). */
{
const UChar* code = guest_code + delta;
/* Spot the 16-byte preamble:
48C1C703 rolq $3, %rdi
48C1C70D rolq $13, %rdi
48C1C73D rolq $61, %rdi
48C1C733 rolq $51, %rdi
*/
if (code[ 0] == 0x48 && code[ 1] == 0xC1 && code[ 2] == 0xC7
&& code[ 3] == 0x03 &&
code[ 4] == 0x48 && code[ 5] == 0xC1 && code[ 6] == 0xC7
&& code[ 7] == 0x0D &&
code[ 8] == 0x48 && code[ 9] == 0xC1 && code[10] == 0xC7
&& code[11] == 0x3D &&
code[12] == 0x48 && code[13] == 0xC1 && code[14] == 0xC7
&& code[15] == 0x33) {
/* Got a "Special" instruction preamble. Which one is it? */
if (code[16] == 0x48 && code[17] == 0x87
&& code[18] == 0xDB /* xchgq %rbx,%rbx */) {
/* %RDX = client_request ( %RAX ) */
DIP("%%rdx = client_request ( %%rax )\n");
delta += 19;
jmp_lit(&dres, Ijk_ClientReq, guest_RIP_bbstart+delta);
vassert(dres.whatNext == Dis_StopHere);
goto decode_success;
}
else