Re: [PATCH 1/4] module: Add module_subinit{_noexit} and module_subeixt helper macros

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Hi, Mika

On 23/07/2024 17:58, Mika Penttilä wrote:
On 7/23/24 11:32, Youling Tang wrote:
From: Youling Tang <tangyouling@xxxxxxxxxx>

In theory init/exit should match their sequence, thus normally they should
look like this:
-------------------------+------------------------
     init_A();            |
     init_B();            |
     init_C();            |
                          |   exit_C();
                          |   exit_B();
                          |   exit_A();

Providing module_subinit{_noexit} and module_subeixt helps macros ensure
that modules init/exit match their order, while also simplifying the code.

The three macros are defined as follows:
- module_subinit(initfn, exitfn,rollback)
- module_subinit_noexit(initfn, rollback)
- module_subexit(rollback)

`initfn` is the initialization function and `exitfn` is the corresponding
exit function.

Signed-off-by: Youling Tang <tangyouling@xxxxxxxxxx>
---
  include/asm-generic/vmlinux.lds.h |  5 +++
  include/linux/init.h              | 62 ++++++++++++++++++++++++++++++-
  include/linux/module.h            | 22 +++++++++++
  3 files changed, 88 insertions(+), 1 deletion(-)

diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
index 677315e51e54..48ccac7c6448 100644
--- a/include/asm-generic/vmlinux.lds.h
+++ b/include/asm-generic/vmlinux.lds.h
@@ -927,6 +927,10 @@
  		INIT_CALLS_LEVEL(7)					\
  		__initcall_end = .;
+#define SUBINIT_CALL \
+		*(.subinitcall.init)					\
+		*(.subexitcall.exit)
+
  #define CON_INITCALL							\
  	BOUNDED_SECTION_POST_LABEL(.con_initcall.init, __con_initcall, _start, _end)
@@ -1155,6 +1159,7 @@
  		INIT_DATA						\
  		INIT_SETUP(initsetup_align)				\
  		INIT_CALLS						\
+		SUBINIT_CALL						\
  		CON_INITCALL						\
  		INIT_RAM_FS						\
  	}
diff --git a/include/linux/init.h b/include/linux/init.h
index ee1309473bc6..e8689ff2cb6c 100644
--- a/include/linux/init.h
+++ b/include/linux/init.h
@@ -55,6 +55,9 @@
  #define __exitdata	__section(".exit.data")
  #define __exit_call	__used __section(".exitcall.exit")
+#define __subinit_call __used __section(".subinitcall.init")
+#define __subexit_call	__used __section(".subexitcall.exit")
+
  /*
   * modpost check for section mismatches during the kernel build.
   * A section mismatch happens when there are references from a
@@ -115,6 +118,9 @@
  typedef int (*initcall_t)(void);
  typedef void (*exitcall_t)(void);
+typedef int (*subinitcall_t)(void);
+typedef void (*subexitcall_t)(void);
+
  #ifdef CONFIG_HAVE_ARCH_PREL32_RELOCATIONS
  typedef int initcall_entry_t;
@@ -183,7 +189,61 @@ extern struct module __this_module;
  #endif
#endif
-
+
+#ifndef __ASSEMBLY__
+struct subexitcall_rollback {
+	/*
+	 * Records the address of the first sub-initialization function in the
+	 * ".subexitcall.exit" section
+	 */
+	unsigned long first_addr;
+	int ncalls;
+};
+
+static inline void __subexitcall_rollback(struct subexitcall_rollback *r)
+{
+	unsigned long addr = r->first_addr - sizeof(r->first_addr) * (r->ncalls - 1);
+
+	for (; r->ncalls--; addr += sizeof(r->first_addr)) {
+		unsigned long *tmp = (void *)addr;
+		subexitcall_t fn = (subexitcall_t)*tmp;
+		fn();
+	}
+}
How does this guarantee the exit calls match sequence? Are you assuming
linker puts exit functions in reverse order?
Take btrfs for example:
Initialize the function sequentially in init_btrfs_fs() using
module_subinit{_noexit}, storing the corresponding function addresses
in the specified ".subinitcall.init" and ".subexitcall.exit" sections.

Using gcc to compile btrfs to.ko, the view section contains the following:
```
$ objdump -d -j ".subinitcall.init" fs/btrfs/super.o

fs/btrfs/super.o:     file format elf64-x86-64


Disassembly of section .subinitcall.init:

0000000000000000 <__subinitcall_register_btrfs.0>:
    ...

0000000000000008 <__subinitcall_btrfs_run_sanity_tests.2>:
    ...

0000000000000010 <__subinitcall_btrfs_print_mod_info.3>:
    ...

0000000000000018 <__subinitcall_btrfs_interface_init.4>:
    ...

0000000000000020 <__subinitcall_btrfs_prelim_ref_init.6>:
    ...

0000000000000028 <__subinitcall_btrfs_delayed_ref_init.8>:
    ...

0000000000000030 <__subinitcall_btrfs_auto_defrag_init.10>:
    ...

0000000000000038 <__subinitcall_btrfs_delayed_inode_init.12>:
    ...

0000000000000040 <__subinitcall_ordered_data_init.14>:
    ...

0000000000000048 <__subinitcall_extent_map_init.16>:
    ...

0000000000000050 <__subinitcall_btrfs_bioset_init.18>:
    ...

0000000000000058 <__subinitcall_extent_buffer_init_cachep.20>:
    ...

0000000000000060 <__subinitcall_extent_state_init_cachep.22>:
    ...

0000000000000068 <__subinitcall_btrfs_free_space_init.24>:
    ...

0000000000000070 <__subinitcall_btrfs_ctree_init.26>:
    ...

0000000000000078 <__subinitcall_btrfs_transaction_init.28>:
    ...

0000000000000080 <__subinitcall_btrfs_init_dio.30>:
    ...

0000000000000088 <__subinitcall_btrfs_init_cachep.32>:
    ...

0000000000000090 <__subinitcall_btrfs_init_compress.34>:
    ...

0000000000000098 <__subinitcall_btrfs_init_sysfs.36>:
    ...

00000000000000a0 <__subinitcall_btrfs_props_init.38>:
    ...

```

```
$ objdump -d -j ".subexitcall.exit" fs/btrfs/super.o

fs/btrfs/super.o:     file format elf64-x86-64


Disassembly of section .subexitcall.exit:

0000000000000000 <__subexitcall_unregister_btrfs.1>:
    ...

0000000000000008 <__subexitcall_btrfs_interface_exit.5>:
    ...

0000000000000010 <__subexitcall_btrfs_prelim_ref_exit.7>:
    ...

0000000000000018 <__subexitcall_btrfs_delayed_ref_exit.9>:
    ...

0000000000000020 <__subexitcall_btrfs_auto_defrag_exit.11>:
    ...

0000000000000028 <__subexitcall_btrfs_delayed_inode_exit.13>:
    ...

0000000000000030 <__subexitcall_ordered_data_exit.15>:
    ...

0000000000000038 <__subexitcall_extent_map_exit.17>:
    ...

0000000000000040 <__subexitcall_btrfs_bioset_exit.19>:
    ...

0000000000000048 <__subexitcall_extent_buffer_free_cachep.21>:
    ...

0000000000000050 <__subexitcall_extent_state_free_cachep.23>:
    ...

0000000000000058 <__subexitcall_btrfs_free_space_exit.25>:
    ...

0000000000000060 <__subexitcall_btrfs_ctree_exit.27>:
    ...

0000000000000068 <__subexitcall_btrfs_transaction_exit.29>:
    ...

0000000000000070 <__subexitcall_btrfs_destroy_dio.31>:
    ...

0000000000000078 <__subexitcall_btrfs_destroy_cachep.33>:
    ...

0000000000000080 <__subexitcall_btrfs_exit_compress.35>:
    ...

0000000000000088 <__subexitcall_btrfs_exit_sysfs.37>:
    ...


```

From the above, we can see that the compiler stores the init/exit function
in reverse order.

Thanks,
Youling.




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