[libc] Update prctl() declaration to use variadic arguments. (#198654)

prctl declaration should typically use variadic arguments (e.g. see
https://man7.org/linux/man-pages/man2/prctl.2.html), as the types /
quantity of subsequent arguments depends on the `option`. We can't
depend on all `<prctl.h>` users to explicitly cast arguments to
`unsigned long` and passing all 5 of them every time.

* Don't add any option-specific logic, and just consume `arg2`-`arg5`
from variadic arguments and pass them to syscall implementation as-is,
assuming that they won't be used by the kernel if they are not needed,
and consuming these arguments won't lead to crashes.
* Updated the test to use `prctl` variants with less than 5 explicit
arguments (for PR_SET_NAME and PR_GET_NAME).

GitOrigin-RevId: 57d2c4c7889333f8d1ebb77efdb5195c2e4f5e1e

include/sys/prctl.yaml

diff --git a/include/sys/prctl.yaml b/include/sys/prctl.yaml
index 53f5764..d8f536f 100644
--- a/include/sys/prctl.yaml
+++ b/include/sys/prctl.yaml
@@ -11,7 +11,4 @@
     return_type: int
     arguments:
       - type: int
-      - type: unsigned long
-      - type: unsigned long
-      - type: unsigned long
-      - type: unsigned long
+      - type: '...'

src/sys/prctl/linux/prctl.cpp

diff --git a/src/sys/prctl/linux/prctl.cpp b/src/sys/prctl/linux/prctl.cpp
index c726b0a..b2e6765 100644
--- a/src/sys/prctl/linux/prctl.cpp
+++ b/src/sys/prctl/linux/prctl.cpp
@@ -12,13 +12,21 @@
 
 #include "src/__support/libc_errno.h"
 #include "src/__support/macros/config.h"
+#include <stdarg.h>
 #include <sys/syscall.h> // For syscall numbers.
 
 namespace LIBC_NAMESPACE_DECL {
 
-LLVM_LIBC_FUNCTION(int, prctl,
-                   (int option, unsigned long arg2, unsigned long arg3,
-                    unsigned long arg4, unsigned long arg5)) {
+LLVM_LIBC_FUNCTION(int, prctl, (int option, ...)) {
+  va_list vargs;
+  va_start(vargs, option);
+  // Try to consume (and pass to syscall_impl) the maximum possible number of
+  // arguments, even though for some "option" values only few will be provided.
+  unsigned long arg2 = va_arg(vargs, unsigned long);
+  unsigned long arg3 = va_arg(vargs, unsigned long);
+  unsigned long arg4 = va_arg(vargs, unsigned long);
+  unsigned long arg5 = va_arg(vargs, unsigned long);
+  va_end(vargs);
   long ret =
       LIBC_NAMESPACE::syscall_impl(SYS_prctl, option, arg2, arg3, arg4, arg5);
   // The manpage states that "... return the nonnegative values described

src/sys/prctl/prctl.h

diff --git a/src/sys/prctl/prctl.h b/src/sys/prctl/prctl.h
index c6b5797..b88c6ab 100644
--- a/src/sys/prctl/prctl.h
+++ b/src/sys/prctl/prctl.h
@@ -14,8 +14,7 @@
 
 namespace LIBC_NAMESPACE_DECL {
 
-int prctl(int option, unsigned long arg2, unsigned long arg3,
-          unsigned long arg4, unsigned long arg5);
+int prctl(int option, ...);
 
 } // namespace LIBC_NAMESPACE_DECL
 

test/src/sys/prctl/linux/prctl_test.cpp

diff --git a/test/src/sys/prctl/linux/prctl_test.cpp b/test/src/sys/prctl/linux/prctl_test.cpp
index 76b829c..0de85b7 100644
--- a/test/src/sys/prctl/linux/prctl_test.cpp
+++ b/test/src/sys/prctl/linux/prctl_test.cpp
@@ -9,25 +9,19 @@
 #include "src/sys/prctl/prctl.h"
 #include "test/UnitTest/ErrnoCheckingTest.h"
 #include "test/UnitTest/ErrnoSetterMatcher.h"
-#include <sys/prctl.h>
+#include <linux/prctl.h> // Definition of PR_* constants
 
 using LIBC_NAMESPACE::testing::ErrnoSetterMatcher::Fails;
 using LIBC_NAMESPACE::testing::ErrnoSetterMatcher::Succeeds;
 using LlvmLibcSysPrctlTest = LIBC_NAMESPACE::testing::ErrnoCheckingTest;
 
 TEST_F(LlvmLibcSysPrctlTest, GetSetName) {
+  ASSERT_THAT(LIBC_NAMESPACE::prctl(PR_GET_NAME, nullptr), Fails(EFAULT, -1));
+
+  ASSERT_THAT(LIBC_NAMESPACE::prctl(PR_SET_NAME, "libc-test"), Succeeds());
+
   char name[17];
-  unsigned long name_addr = 0;
-  ASSERT_THAT(LIBC_NAMESPACE::prctl(PR_GET_NAME, name_addr, 0, 0, 0),
-              Fails(EFAULT, -1));
-
-  name_addr = reinterpret_cast<unsigned long>("libc-test");
-  ASSERT_THAT(LIBC_NAMESPACE::prctl(PR_SET_NAME, name_addr, 0, 0, 0),
-              Succeeds());
-
-  name_addr = reinterpret_cast<unsigned long>(name);
-  ASSERT_THAT(LIBC_NAMESPACE::prctl(PR_GET_NAME, name_addr, 0, 0, 0),
-              Succeeds());
+  ASSERT_THAT(LIBC_NAMESPACE::prctl(PR_GET_NAME, name), Succeeds());
   ASSERT_STREQ(name, "libc-test");
 }