[RFC PATCH v1 08/57] fs: Remove PAGE_SIZE compile-time constant assumption

[Ryan Roberts]

To prepare for supporting boot-time page size selection, refactor code
to remove assumptions about PAGE_SIZE being compile-time constant. Code
intended to be equivalent when compile-time page size is active.

In binfmt_elf, convert CPP conditional to C ternary operator; this will
be folded to the same code by the compiler when in compile-time page
size mode, but will also work for runtime evaluation in boot-time page
size mode.

In coredump, modify __dump_skip() to emit zeros in blocks of
PAGE_SIZE_MIN. This resolves to the previous PAGE_SIZE for compile-time
page size, but that doesn't work for boot-time page size. PAGE_SIZE_MIN
is preferred here over PAGE_SIZE_MAX to save memory.

Wrap global variables that are initialized with PAGE_SIZE derived values
using DEFINE_GLOBAL_PAGE_SIZE_VAR() so their initialization can be
deferred for boot-time page size builds.

Signed-off-by: Ryan Roberts <ryan.roberts@xxxxxxx>
---

***NOTE***
Any confused maintainers may want to read the cover note here for context:
https://lore.kernel.org/all/20241014105514.3206191-1-ryan.roberts@xxxxxxx/

 fs/binfmt_elf.c | 11 ++++-------
 fs/coredump.c   |  8 ++++----
 2 files changed, 8 insertions(+), 11 deletions(-)

diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
index 19fa49cd9907f..e439d36c43c7e 100644
--- a/fs/binfmt_elf.c
+++ b/fs/binfmt_elf.c
@@ -84,11 +84,8 @@ static int elf_core_dump(struct coredump_params *cprm);
 #define elf_core_dump	NULL
 #endif
 
-#if ELF_EXEC_PAGESIZE > PAGE_SIZE
-#define ELF_MIN_ALIGN	ELF_EXEC_PAGESIZE
-#else
-#define ELF_MIN_ALIGN	PAGE_SIZE
-#endif
+#define ELF_MIN_ALIGN							\
+	(ELF_EXEC_PAGESIZE > PAGE_SIZE ? ELF_EXEC_PAGESIZE : PAGE_SIZE)
 
 #ifndef ELF_CORE_EFLAGS
 #define ELF_CORE_EFLAGS	0
@@ -98,7 +95,7 @@ static int elf_core_dump(struct coredump_params *cprm);
 #define ELF_PAGEOFFSET(_v) ((_v) & (ELF_MIN_ALIGN-1))
 #define ELF_PAGEALIGN(_v) (((_v) + ELF_MIN_ALIGN - 1) & ~(ELF_MIN_ALIGN - 1))
 
-static struct linux_binfmt elf_format = {
+static DEFINE_GLOBAL_PAGE_SIZE_VAR(struct linux_binfmt, elf_format, {
 	.module		= THIS_MODULE,
 	.load_binary	= load_elf_binary,
 	.load_shlib	= load_elf_library,
@@ -106,7 +103,7 @@ static struct linux_binfmt elf_format = {
 	.core_dump	= elf_core_dump,
 	.min_coredump	= ELF_EXEC_PAGESIZE,
 #endif
-};
+});
 
 #define BAD_ADDR(x) (unlikely((unsigned long)(x) >= TASK_SIZE))
 
diff --git a/fs/coredump.c b/fs/coredump.c
index 7f12ff6ad1d3e..203f2a158246e 100644
--- a/fs/coredump.c
+++ b/fs/coredump.c
@@ -825,7 +825,7 @@ static int __dump_emit(struct coredump_params *cprm, const void *addr, int nr)
 
 static int __dump_skip(struct coredump_params *cprm, size_t nr)
 {
-	static char zeroes[PAGE_SIZE];
+	static char zeroes[PAGE_SIZE_MIN];
 	struct file *file = cprm->file;
 	if (file->f_mode & FMODE_LSEEK) {
 		if (dump_interrupted() ||
@@ -834,10 +834,10 @@ static int __dump_skip(struct coredump_params *cprm, size_t nr)
 		cprm->pos += nr;
 		return 1;
 	} else {
-		while (nr > PAGE_SIZE) {
-			if (!__dump_emit(cprm, zeroes, PAGE_SIZE))
+		while (nr > PAGE_SIZE_MIN) {
+			if (!__dump_emit(cprm, zeroes, PAGE_SIZE_MIN))
 				return 0;
-			nr -= PAGE_SIZE;
+			nr -= PAGE_SIZE_MIN;
 		}
 		return __dump_emit(cprm, zeroes, nr);
 	}
-- 
2.43.0