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Subject[PATCH] exec: move struct linux_binprm::buf
struct linux_binprm::buf is the first field and it is exactly 128 bytes
in size. It means that on x86_64 all accesses to other fields will go
though [r64 + disp32] addressing mode which is 3 bytes bloatier than
[r64 + disp8] addressing mode. Given that accesses to other fields
outnumber accesses to ->buf, move it down.

Space savings (x86_64 defconfig):
more on distro configs because LSMs actively dereference "bprm"
but do not care about first 128 bytes of the executable itself.

add/remove: 0/0 grow/shrink: 0/24 up/down: 0/-492 (-492)
Function old new delta
selinux_bprm_committing_creds 552 549 -3
finalize_exec 94 91 -3
__audit_log_bprm_fcaps 283 280 -3
__audit_bprm 39 36 -3
perf_trace_sched_process_exec 347 341 -6
install_exec_creds 105 99 -6
cap_bprm_set_creds.cold 60 54 -6
would_dump 137 128 -9
load_script 637 628 -9
bprm_change_interp 61 52 -9
trace_event_raw_event_sched_process_exec 260 250 -10
search_binary_handler 255 240 -15
remove_arg_zero 295 277 -18
free_bprm 119 101 -18
prepare_binprm 379 360 -19
setup_new_exec 336 315 -21
flush_old_exec 1638 1617 -21
copy_strings.isra 746 724 -22
setup_arg_pages 559 530 -29
load_misc_binary 1151 1118 -33
selinux_bprm_set_creds 792 753 -39
load_elf_binary 11111 11072 -39
cap_bprm_set_creds 1496 1454 -42
__do_execve_file.isra 2395 2286 -109

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
---

include/linux/binfmts.h | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)

--- a/include/linux/binfmts.h
+++ b/include/linux/binfmts.h
@@ -15,7 +15,6 @@ struct filename;
* This structure is used to hold the arguments that are used when loading binaries.
*/
struct linux_binprm {
- char buf[BINPRM_BUF_SIZE];
#ifdef CONFIG_MMU
struct vm_area_struct *vma;
unsigned long vma_pages;
@@ -64,6 +63,8 @@ struct linux_binprm {
unsigned long loader, exec;

struct rlimit rlim_stack; /* Saved RLIMIT_STACK used during exec. */
+
+ char buf[BINPRM_BUF_SIZE];
} __randomize_layout;

#define BINPRM_FLAGS_ENFORCE_NONDUMP_BIT 0
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