lkl: Add a config option to select section for kallsyms

This commit adds a kernel config option to select whether the
kallsyms data should be in the .rodata section (the default for
non-LKL builds), or in the .data section (the default for LKL).

This is to avoid relocations in the text segment (TEXTRELs) that
would otherwise occur with LKL when the .rodata and the .text
section end up in the same segment.

Having TEXTRELs can lead to a number of issues:

1. If a shared library contains a TEXTREL, the corresponding memory
pages cannot be shared.

2. Android >=6 and SELinux do not support binaries with TEXTRELs
(http://android-developers.blogspot.com/2016/06/android-changes-for-ndk-developers.html).

3. If a program has a TEXTREL, uses an ifunc, and is compiled with
early binding, this can lead to a segmentation fault when processing
the relocation for the ifunc during dynamic linking because the text
segment is made temporarily non-executable to process the TEXTREL
(line 248 in dl_reloc.c).

Signed-off-by: Andreas Abel <[email protected]>

arch/lkl/defconfig

@@ -1,6 +1,7 @@
 # CONFIG_LOCALVERSION_AUTO is not set
 CONFIG_NO_HZ_IDLE=y
 # CONFIG_SYSFS_SYSCALL is not set
+CONFIG_KALLSYMS_USE_DATA_SECTION=y
 CONFIG_KALLSYMS_ALL=y
 # CONFIG_BASE_FULL is not set
 # CONFIG_FUTEX is not set

init/Kconfig

@@ -1443,6 +1443,18 @@ config KALLSYMS_BASE_RELATIVE
 	  time constants, and no relocation pass is required at runtime to fix
 	  up the entries based on the runtime load address of the kernel.
 
+config KALLSYMS_USE_DATA_SECTION
+	bool "Use .data instead of .rodata section for kallsyms"
+	depends on KALLSYMS
+	default n
+	help
+	  Enabling this option will put the kallsyms data in the .data section
+	  instead of the .rodata section.
+
+	  This is useful when building the kernel as a library, as it avoids
+	  relocations in the text segment that could otherwise occur if the
+	  .rodata section is in the same segment as the .text section.
+
 config PRINTK
 	default y
 	bool "Enable support for printk" if EXPERT

scripts/kallsyms.c

@@ -60,11 +60,12 @@ static struct addr_range percpu_range = {
 
 static struct sym_entry *table;
 static unsigned int table_size, table_cnt;
-static int all_symbols = 0;
-static int absolute_percpu = 0;
+static int all_symbols;
+static int use_data_section;
+static int absolute_percpu;
 static char symbol_prefix_char = '\0';
-static unsigned long long kernel_start_addr = 0;
-static int base_relative = 0;
+static unsigned long long kernel_start_addr;
+static int base_relative;
 
 int token_profit[0x10000];
 
@@ -76,6 +77,7 @@ unsigned char best_table_len[256];
 static void usage(void)
 {
 	fprintf(stderr, "Usage: kallsyms [--all-symbols] "
+			"[--use-data-section] "
 			"[--symbol-prefix=<prefix char>] "
 			"[--page-offset=<CONFIG_PAGE_OFFSET>] "
 			"[--base-relative] < in.map > out.S\n");
@@ -352,7 +354,10 @@ static void write_src(void)
 	printf("#define ALGN .align 4\n");
 	printf("#endif\n");
 
-	printf("\t.section .rodata, \"a\"\n");
+	if (use_data_section)
+		printf("\t.section .data\n");
+	else
+		printf("\t.section .rodata, \"a\"\n");
 
 	/* Provide proper symbols relocatability by their relativeness
 	 * to a fixed anchor point in the runtime image, either '_text'
@@ -759,6 +764,8 @@ int main(int argc, char **argv)
 				all_symbols = 1;
 			else if (strcmp(argv[i], "--absolute-percpu") == 0)
 				absolute_percpu = 1;
+			else if (strcmp(argv[i], "--use-data-section") == 0)
+				use_data_section = 1;
 			else if (strncmp(argv[i], "--symbol-prefix=", 16) == 0) {
 				char *p = &argv[i][16];
 				/* skip quote */

scripts/link-vmlinux.sh

@@ -94,6 +94,10 @@ kallsyms()
 		kallsymopt="${kallsymopt} --base-relative"
 	fi
 
+	if [ -n "${CONFIG_KALLSYMS_USE_DATA_SECTION}" ]; then
+		kallsymopt="${kallsymopt} --use-data-section"
+	fi
+
 	local aflags="${KBUILD_AFLAGS} ${KBUILD_AFLAGS_KERNEL}               \
 		      ${NOSTDINC_FLAGS} ${LINUXINCLUDE} ${KBUILD_CPPFLAGS}"