bpf, x86: Make sure allocation in arch_bpf_trampoline_size() is writable

arch_bpf_trampoline_size() allocates a buffer to get actual size required
for a trampoline.

This buffer must be in the module address space because
__arch_prepare_bpf_trampoline() calculates  rel32 offsets relatively to
that buffer.

In preparation for enabling ROX mode for EXECMEM_BPF make sure that the
allocated memory is writable.

Add bpf_jit_alloc_exec_rw() wrapper for execmem_alloc_rw() and use it for
buffer allocation in arch_bpf_trampoline_size().

Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Acked-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/bpf/20260716-execmem-x86-rox-bpf-v0-v3-4-4e76158c01c5@kernel.org
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>

arch/x86/net/bpf_jit_comp.c

@@ -3703,13 +3703,12 @@ int arch_bpf_trampoline_size(const struct btf_func_model *m, u32 flags,
 	int ret;
 
 	/* Allocate a temporary buffer for __arch_prepare_bpf_trampoline().
-	 * This will NOT cause fragmentation in direct map, as we do not
-	 * call set_memory_*() on this buffer.
 	 *
 	 * We cannot use kvmalloc here, because we need image to be in
 	 * module memory range.
+	 * Since it must be writable use bpf_jit_alloc_exec_rw().
 	 */
-	image = bpf_jit_alloc_exec(PAGE_SIZE);
+	image = bpf_jit_alloc_exec_rw(PAGE_SIZE);
 	if (!image)
 		return -ENOMEM;
 

include/linux/filter.h

@@ -1333,6 +1333,7 @@ bpf_jit_binary_alloc(unsigned int proglen, u8 **image_ptr,
 void bpf_jit_binary_free(struct bpf_binary_header *hdr);
 u64 bpf_jit_alloc_exec_limit(void);
 void *bpf_jit_alloc_exec(unsigned long size);
+void *bpf_jit_alloc_exec_rw(unsigned long size);
 void bpf_jit_free_exec(void *addr);
 void bpf_jit_free(struct bpf_prog *fp);
 struct bpf_binary_header *

kernel/bpf/core.c

@@ -1128,6 +1128,11 @@ void *bpf_jit_alloc_exec(unsigned long size)
 	return execmem_alloc(EXECMEM_BPF, size);
 }
 
+void *bpf_jit_alloc_exec_rw(unsigned long size)
+{
+	return execmem_alloc_rw(EXECMEM_BPF, size);
+}
+
 void bpf_jit_free_exec(void *addr)
 {
 	execmem_free(addr);