Re: [PATCH v9 3/6] module: use relative references for __ksymtab entries

On 27 June 2018 at 17:13, Will Deacon [off-list ref] wrote:

Hi Ard,

On Tue, Jun 26, 2018 at 08:27:58PM +0200, Ard Biesheuvel wrote:

quoted

An ordinary arm64 defconfig build has ~64 KB worth of __ksymtab
entries, each consisting of two 64-bit fields containing absolute
references, to the symbol itself and to a char array containing
its name, respectively.

[...]

quoted

diff --git a/include/linux/export.h b/include/linux/export.h
index ea7df303d68d..ae072bc5aacf 100644
--- a/include/linux/export.h
+++ b/include/linux/export.h

@@ -18,12 +18,6 @@
 #define VMLINUX_SYMBOL_STR(x) __VMLINUX_SYMBOL_STR(x)

 #ifndef __ASSEMBLY__
-struct kernel_symbol
-{
-     unsigned long value;
-     const char *name;
-};
-
 #ifdef MODULE
 extern struct module __this_module;
 #define THIS_MODULE (&__this_module)

@@ -54,17 +48,47 @@ extern struct module __this_module;
 #define __CRC_SYMBOL(sym, sec)
 #endif

+#ifdef CONFIG_HAVE_ARCH_PREL32_RELOCATIONS
+#include <linux/compiler.h>
+/*
+ * Emit the ksymtab entry as a pair of relative references: this reduces
+ * the size by half on 64-bit architectures, and eliminates the need for
+ * absolute relocations that require runtime processing on relocatable
+ * kernels.
+ */
+#define __KSYMTAB_ENTRY(sym, sec)                                    \
+     __ADDRESSABLE(sym)                                              \
+     asm("   .section \"___ksymtab" sec "+" #sym "\", \"a\"  \n"     \
+         "   .balign 8                                       \n"     \

Can we use KSYM_ALIGN here instead of 8, or do we need the 8-byte alignment
even on 32-bit architectures?

We don't *need* 8 byte alignment on any architecture, but since the
structure itself is 8 bytes in size and we have a sizable array of
them, it makes sense to align them to 8 bytes.