arm64: module: mandate MODULE_PLTS

Contemporary kernels and modules can be relatively large, especially
when common debug options are enabled. Using GCC 12.1.0, a v6.3-rc7
defconfig kernel is ~38M, and with PROVE_LOCKING + KASAN_INLINE enabled
this expands to ~117M. Shanker reports [1] that the NVIDIA GPU driver
alone can consume 110M of module space in some configurations.

Both KASLR and ARM64_ERRATUM_843419 select MODULE_PLTS, so anyone
wanting a kernel to have KASLR or run on Cortex-A53 will have
MODULE_PLTS selected. This is the case in defconfig and distribution
kernels (e.g. Debian, Android, etc).

Practically speaking, this means we're very likely to need MODULE_PLTS
and while it's almost guaranteed that MODULE_PLTS will be selected, it
is possible to disable support, and we have to maintain some awkward
special cases for such unusual configurations.

This patch removes the MODULE_PLTS config option, with the support code
always enabled if MODULES is selected. This results in a slight
simplification, and will allow for further improvement in subsequent
patches.

For any config which currently selects MODULE_PLTS, there will be no
functional change as a result of this patch.

[1] https://lore.kernel.org/linux-arm-kernel/159ceeab-09af-3174-5058-445bc8dcf85b@nvidia.com/

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
Cc: Shanker Donthineni <sdonthineni@nvidia.com>
Cc: Will Deacon <will@kernel.org>
Tested-by: Shanker Donthineni <sdonthineni@nvidia.com>
Link: https://lore.kernel.org/r/20230530110328.2213762-6-mark.rutland@arm.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

1 files changed, 12 insertions, 13 deletions

diff --git a/arch/arm64/kernel/module.c b/arch/arm64/kernel/module.c
index 51fd8abd9b74..f64636c2fd05 100644
--- a/arch/arm64/kernel/module.c
+++ b/arch/arm64/kernel/module.c
@@ -73,25 +73,26 @@ subsys_initcall(kaslr_module_init)
 void *module_alloc(unsigned long size)
 {
 	u64 module_alloc_end = module_alloc_base + MODULES_VSIZE;
-	gfp_t gfp_mask = GFP_KERNEL;
 	void *p;
 
-	/* Silence the initial allocation */
-	if (IS_ENABLED(CONFIG_ARM64_MODULE_PLTS))
-		gfp_mask |= __GFP_NOWARN;
-
+	/*
+	 * Where possible, prefer to allocate within direct branch range of the
+	 * kernel such that no PLTs are necessary. This may fail, so we pass
+	 * __GFP_NOWARN to silence the resulting warning.
+	 */
 	p = __vmalloc_node_range(size, MODULE_ALIGN, module_alloc_base,
-				module_alloc_end, gfp_mask, PAGE_KERNEL, 0,
-				NUMA_NO_NODE, __builtin_return_address(0));
+				module_alloc_end, GFP_KERNEL | __GFP_NOWARN,
+				PAGE_KERNEL, 0, NUMA_NO_NODE,
+				__builtin_return_address(0));
 
-	if (!p && IS_ENABLED(CONFIG_ARM64_MODULE_PLTS)) {
+	if (!p) {
 		p = __vmalloc_node_range(size, MODULE_ALIGN, module_alloc_base,
 				module_alloc_base + SZ_2G, GFP_KERNEL,
 				PAGE_KERNEL, 0, NUMA_NO_NODE,
 				__builtin_return_address(0));
 	}
 
-	if (p && (kasan_alloc_module_shadow(p, size, gfp_mask) < 0)) {
+	if (p && (kasan_alloc_module_shadow(p, size, GFP_KERNEL) < 0)) {
 		vfree(p);
 		return NULL;
 	}
@@ -479,9 +480,7 @@ int apply_relocate_add(Elf64_Shdr *sechdrs,
 		case R_AARCH64_CALL26:
 			ovf = reloc_insn_imm(RELOC_OP_PREL, loc, val, 2, 26,
 					     AARCH64_INSN_IMM_26);
-
-			if (IS_ENABLED(CONFIG_ARM64_MODULE_PLTS) &&
-			    ovf == -ERANGE) {
+			if (ovf == -ERANGE) {
 				val = module_emit_plt_entry(me, sechdrs, loc, &rel[i], sym);
 				if (!val)
 					return -ENOEXEC;
@@ -518,7 +517,7 @@ static int module_init_ftrace_plt(const Elf_Ehdr *hdr,
 				  const Elf_Shdr *sechdrs,
 				  struct module *mod)
 {
-#if defined(CONFIG_ARM64_MODULE_PLTS) && defined(CONFIG_DYNAMIC_FTRACE)
+#if defined(CONFIG_DYNAMIC_FTRACE)
 	const Elf_Shdr *s;
 	struct plt_entry *plts;