fix: real argv[0] via SYS_GETEXECPATH - GCC driver finds its backends

The GCC driver relocates its install prefix from argv[0]. crt1
hardcoded argv[0] as "prog", so make_relative_prefix cwd-joined it
and computed exec prefixes relative to the current directory
(0:/users/admin/../libexec/gcc/...), and because the computed
gcc_exec_prefix is non-NULL the standard /sdk prefixes were never
searched. cc1plus was unreachable from anywhere except (by accident
of the path arithmetic) 0:/sdk/bin. sdk-diag proved the kernel and
libc layers all worked; only the driver's self-relocation was lost.

New SYS_GETEXECPATH (151) returns the absolute path the process was
spawned from (Process::name); crt1 uses it for argv[0] with a "prog"
fallback. With a real argv[0], make_relative_prefix computes
0:/sdk/bin/../libexec/gcc/ from any cwd. Native GCC relinked against
the new crt1; sdk-diag ships in the SDK as a permanent probe; the
montauk.h TCC mirror gains the wrapper (checker enforced it).

Co-Authored-By: Claude Fable 5 <[email protected]>

kernel/src/Api/BuildNo.hpp

@@ -12,4 +12,4 @@
 
 #pragma once
 
-#define MONTAUK_BUILD_NUMBER 23
+#define MONTAUK_BUILD_NUMBER 25

kernel/src/Api/Process.hpp

@@ -84,6 +84,18 @@ namespace montauk::abi {
         return childPid;
     }
 
+    // Copy the absolute path this process was spawned from (argv[0]).
+    static int Sys_GetExecPath(char* buf, uint64_t maxLen) {
+        auto* proc = Sched::GetCurrentProcessPtr();
+        if (proc == nullptr || buf == nullptr || maxLen == 0) return -1;
+        int i = 0;
+        for (; i < (int)maxLen - 1 && proc->name[i]; i++) {
+            buf[i] = proc->name[i];
+        }
+        buf[i] = '\0';
+        return i;
+    }
+
     static int Sys_GetArgs(char* buf, uint64_t maxLen) {
         auto* proc = Sched::GetCurrentProcessPtr();
         if (proc == nullptr || buf == nullptr || maxLen == 0) return -1;

kernel/src/Api/Syscall.cpp

@@ -150,6 +150,9 @@ namespace montauk::abi {
                 return (int64_t)Sys_FbMap();
             case SYS_FBFLIP:
                 return Sys_FbFlip(frame->arg1, frame->arg2);
+            case SYS_GETEXECPATH:
+                if (!UserMemory::Range(frame->arg2 ? frame->arg1 : frame->arg1, frame->arg2, true)) return -1;
+                return Sys_GetExecPath((char*)frame->arg1, frame->arg2);
             case SYS_TERMSIZE:
                 return (int64_t)Sys_TermSize();
             case SYS_GETARGS:

kernel/src/Api/Syscall.hpp

@@ -280,7 +280,10 @@ namespace montauk::abi {
     static constexpr uint64_t SYS_POWERINFO           = 149;  // (PowerInfo*) -> 0, -1 unsupported
 
     /* Graphics.hpp -- framebuffer page flip (double-buffered scanout) */
-    static constexpr uint64_t SYS_FBFLIP              = 150;  // (index, flags) -> new front index; index=-1 queries support (1/0); flags bit0 = wait vsync
+    static constexpr uint64_t SYS_FBFLIP              = 150;
+
+    // Absolute path of the running executable (for argv[0]).
+    static constexpr uint64_t SYS_GETEXECPATH         = 151;  // (index, flags) -> new front index; index=-1 queries support (1/0); flags bit0 = wait vsync
 
     // Tunable parameters (for SYS_SDR_SETPARAM / SYS_SDR_GETPARAM).
     static constexpr int SDR_PARAM_FREQ        = 0;  // center frequency, Hz