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

Ryan Roberts
2024-10-14T11:34:56Z

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@arm.com>
---

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

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