[RFC PATCH 4/9] module: Allocate MOD_INIT_TEXT from the MOD_TEXT ROX allocation

[Ard Biesheuvel <ardb+git@google.com>]

From: Ard Biesheuvel <ardb@kernel.org>

When execmem ROX caches are used for module text and inittext, place
them adjacently in memory, by allocating space for both initially, and
splitting off the space for MOD_INIT_TEXT as needed.

This avoids the corner case on arm64, where .init.text being placed far
from .text results in a lot of complexity wrt indirect branches and PLTs
that we'd prefer to avoid.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
 kernel/module/main.c | 22 ++++++++++++++++----
 1 file changed, 18 insertions(+), 4 deletions(-)

diff --git a/kernel/module/main.c b/kernel/module/main.c
index 46dd8d25a605..2d6213496359 100644
--- a/kernel/module/main.c
+++ b/kernel/module/main.c
@@ -1342,7 +1342,7 @@ static int module_memory_alloc(struct module *mod, enum mod_mem_type type)
 {
 	unsigned int size = PAGE_ALIGN(mod->mem[type].size);
 	enum execmem_type execmem_type;
-	void *ptr;
+	void *ptr = NULL;
 
 	mod->mem[type].size = size;
 
@@ -1351,11 +1351,25 @@ static int module_memory_alloc(struct module *mod, enum mod_mem_type type)
 	else
 		execmem_type = EXECMEM_MODULE_TEXT;
 
-	ptr = execmem_alloc_rw(execmem_type, size);
+	bool is_rox = execmem_is_rox(execmem_type);
+	if (is_rox) {
+		/*
+		 * Special case for MOD_TEXT / MOD_INIT_TEXT: allocate the
+		 * latter by splitting off required space from the former
+		 * so that they are always placed close together.
+		 */
+		if (type == MOD_TEXT)
+			size += PAGE_ALIGN(mod->mem[MOD_INIT_TEXT].size);
+		else if (type == MOD_INIT_TEXT)
+			ptr = execmem_split(mod->mem[MOD_TEXT].base, size);
+	}
+
+	if (!ptr)
+		ptr = execmem_alloc_rw(execmem_type, size);
 	if (!ptr)
 		return -ENOMEM;
 
-	mod->mem[type].is_rox = execmem_is_rox(execmem_type);
+	mod->mem[type].is_rox = is_rox;
 
 	/*
 	 * The pointer to these blocks of memory are stored on the module
@@ -1368,7 +1382,7 @@ static int module_memory_alloc(struct module *mod, enum mod_mem_type type)
 	 * *do* eventually get freed, but let's just keep things simple
 	 * and avoid *any* false positives.
 	 */
-	if (!mod->mem[type].is_rox)
+	if (!is_rox)
 		kmemleak_not_leak(ptr);
 
 	memset(ptr, 0, size);
-- 
2.55.0.860.g4b6b3295ed-goog