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

programs/GNUmakefile

@@ -405,6 +405,7 @@ ifneq ($(wildcard $(NATIVE_BIN)/as),)
 	cp -r ../toolchain/local/x86_64-montauk/include/c++ $(BINDIR)/sdk/include/c++
 	../toolchain/local/bin/x86_64-montauk-gcc -O2 ../toolchain/files/tls-test.c -o $(BINDIR)/sdk/bin/tls-test.elf
 	../toolchain/local/bin/x86_64-montauk-g++ -O2 ../toolchain/files/cxx-test.cpp -o $(BINDIR)/sdk/bin/cxx-test.elf
+	../toolchain/local/bin/x86_64-montauk-gcc -O2 ../toolchain/files/sdk-diag.c -o $(BINDIR)/sdk/bin/sdk-diag.elf
 ifneq ($(wildcard ../toolchain/native-gcc/sdk/bin/gcc),)
 	cp ../toolchain/native-gcc/sdk/bin/gcc $(BINDIR)/sdk/bin/gcc.elf
 	cp ../toolchain/native-gcc/sdk/bin/g++ $(BINDIR)/sdk/bin/g++.elf

programs/include/Api/Syscall.hpp

@@ -204,7 +204,10 @@ namespace montauk::abi {
     static constexpr uint64_t SYS_POWERINFO           = 149;  // (PowerInfo*) -> 0, -1 unsupported
 
     // 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

programs/include/libc/montauk.h

@@ -174,6 +174,7 @@ extern "C" {
 #define MTK_SYS_SDR_GETPARAM       148
 #define MTK_SYS_POWERINFO          149
 #define MTK_SYS_FBFLIP             150
+#define MTK_SYS_GETEXECPATH        151
 /* @SYSCALLS-END */
 
 #define MTK_SOCK_TCP 1
@@ -410,6 +411,10 @@ static inline int mtk_getargs(char *buf, unsigned long max_len) {
     return (int)_mtk_syscall2(MTK_SYS_GETARGS, (long)buf, (long)max_len);
 }
 
+static inline int mtk_getexecpath(char *buf, unsigned long maxLen) {
+    return (int)mtk_syscall2(MTK_SYS_GETEXECPATH, (long)buf, (long)maxLen);
+}
+
 static inline int mtk_chdir(const char *path) {
     return (int)_mtk_syscall1(MTK_SYS_CHDIR, (long)path);
 }

programs/lib/libc/crt/crt1.c

@@ -31,8 +31,9 @@ static inline long _sys2(long nr, long a1, long a2) {
     return ret;
 }
 
-#define SYS_EXIT    0
+#define SYS_EXIT        0
-#define SYS_GETARGS 25
+#define SYS_GETARGS     25
+#define SYS_GETEXECPATH 151
 
 extern int main(int argc, char** argv);
 
@@ -71,12 +72,17 @@ static void _run_fini_array(void) {
 void _start(void) {
     /* Static: 4 KiB args + 256 argv slots would crowd a 32 KiB stack. */
     static char argbuf[4096];
+    static char pathbuf[256];
     static char* argv[256];
     int len = (int)_sys2(SYS_GETARGS, (long)argbuf, (long)sizeof(argbuf));
 
     int argc = 0;
 
-    argv[argc++] = (char*)"prog";
+    /* Real argv[0]: tools like the GCC driver relocate their install
+       prefix from it, so "prog" placeholders send them to garbage
+       paths. Fall back to "prog" on kernels without the syscall. */
+    int plen = (int)_sys2(SYS_GETEXECPATH, (long)pathbuf, (long)sizeof(pathbuf));
+    argv[argc++] = (plen > 0) ? pathbuf : (char*)"prog";
 
     if (len > 0) {
         if (len > (int)sizeof(argbuf) - 1) {

toolchain/files/sdk-diag.c

@@ -0,0 +1,44 @@
+/*
+    * sdk-diag.c
+    * Probes the exact libc/kernel layers the GCC driver depends on:
+    * access/stat on the compiler backends, and a direct posix_spawn
+    * of cc1plus. Prints one line per probe.
+*/
+
+#include <stdio.h>
+#include <unistd.h>
+#include <sys/stat.h>
+#include <spawn.h>
+#include <sys/wait.h>
+
+#define CC1PLUS "/sdk/libexec/gcc/x86_64-montauk/14.2.0/cc1plus"
+
+static void probe(const char *p) {
+    struct stat st;
+    int a = access(p, X_OK);
+    int s = stat(p, &st);
+    printf("%-52s access=%d stat=%d", p, a, s);
+    if (s == 0) {
+        printf(" mode=%o size=%ld", (unsigned)st.st_mode, (long)st.st_size);
+    }
+    printf("\n");
+}
+
+int main(void) {
+    probe(CC1PLUS);
+    probe("/sdk/libexec/gcc/x86_64-montauk/14.2.0");
+    probe("/sdk/libexec/gcc");
+    probe("/sdk/bin/as.elf");
+    probe("/tmp");
+
+    pid_t pid = -1;
+    char *argv[] = { (char *)"cc1plus", (char *)"--version", 0 };
+    int r = posix_spawn(&pid, CC1PLUS, 0, 0, argv, 0);
+    printf("posix_spawn(cc1plus --version) = %d pid=%d\n", r, (int)pid);
+    if (r == 0) {
+        int st = -1;
+        waitpid(pid, &st, 0);
+        printf("cc1plus wait status = %d (exit %d)\n", st, (st >> 8) & 0xFF);
+    }
+    return 0;
+}