[PATCH liburing v2 5/8] arch/aarch64: lib: Add `get_page_size()` function

[Ammar Faizi <ammarfaizi2@gnuweeb.org>]

From: Ammar Faizi <ammarfaizi2@gnuweeb.org>

A prep patch to add aarch64 nolibc support.

aarch64 supports three values of page size: 4K, 16K, and 64K which are
selected at kernel compilation time. Therefore, we can't hard code the
page size for this arch. Utilize open(), read() and close() syscall to
find the page size from /proc/self/auxv. For more details about the
auxv data structure, check the link below [1].

v2:
  - Fallback to 4K if the syscall fails.
  - Cache the page size after read as suggested by Jens.

Link: https://github.com/torvalds/linux/blob/v5.19-rc4/fs/binfmt_elf.c#L260 [1]
Link: https://lore.kernel.org/io-uring/3895dbe1-8d5f-cf53-e94b-5d1545466de1@kernel.dk
Link: https://lore.kernel.org/io-uring/8bfba71c-55d7-fb49-6593-4d0f9d9c3611@kernel.dk
Suggested-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Ammar Faizi <ammarfaizi2@gnuweeb.org>
---
 src/arch/aarch64/lib.h | 45 ++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 45 insertions(+)
 create mode 100644 src/arch/aarch64/lib.h

diff --git a/src/arch/aarch64/lib.h b/src/arch/aarch64/lib.h
new file mode 100644
index 0000000..adba512
--- /dev/null
+++ b/src/arch/aarch64/lib.h
@@ -0,0 +1,45 @@
+/* SPDX-License-Identifier: MIT */
+
+#ifndef LIBURING_ARCH_AARCH64_LIB_H
+#define LIBURING_ARCH_AARCH64_LIB_H
+
+#include <elf.h>
+#include <sys/auxv.h>
+#include "../../syscall.h"
+
+static inline long get_page_size(void)
+{
+	static const long fallback_ret = 4096;
+	static long cache_val = 0;
+	Elf64_Off buf[2];
+	long page_size;
+	int fd;
+
+	if (cache_val)
+		return cache_val;
+
+	fd = __sys_open("/proc/self/auxv", O_RDONLY, 0);
+	if (fd < 0)
+		return fallback_ret;
+
+	while (1) {
+		ssize_t x;
+
+		x = __sys_read(fd, buf, sizeof(buf));
+		if (x < sizeof(buf)) {
+			page_size = fallback_ret;
+			break;
+		}
+
+		if (buf[0] == AT_PAGESZ) {
+			page_size = buf[1];
+			cache_val = page_size;
+			break;
+		}
+	}
+
+	__sys_close(fd);
+	return page_size;
+}
+
+#endif /* #ifndef LIBURING_ARCH_AARCH64_LIB_H */
-- 
Ammar Faizi