[PATCH RFC] binfmt_elf: fully allocate bss pages

[Thomas Weißschuh]

When allocating the pages for bss the start address needs to be rounded
down instead of up.
Otherwise the start of the bss segment may be unmapped.

The was reported to happen on Aarch64:

Memory allocated by set_brk():
Before: start=0x420000 end=0x420000
After:  start=0x41f000 end=0x420000

The triggering binary looks like this:

    Elf file type is EXEC (Executable file)
    Entry point 0x400144
    There are 4 program headers, starting at offset 64

    Program Headers:
      Type           Offset             VirtAddr           PhysAddr
                     FileSiz            MemSiz              Flags  Align
      LOAD           0x0000000000000000 0x0000000000400000 0x0000000000400000
                     0x0000000000000178 0x0000000000000178  R E    0x10000
      LOAD           0x000000000000ffe8 0x000000000041ffe8 0x000000000041ffe8
                     0x0000000000000000 0x0000000000000008  RW     0x10000
      NOTE           0x0000000000000120 0x0000000000400120 0x0000000000400120
                     0x0000000000000024 0x0000000000000024  R      0x4
      GNU_STACK      0x0000000000000000 0x0000000000000000 0x0000000000000000
                     0x0000000000000000 0x0000000000000000  RW     0x10

     Section to Segment mapping:
      Segment Sections...
       00     .note.gnu.build-id .text .eh_frame
       01     .bss
       02     .note.gnu.build-id
       03

Reported-by: Sebastian Ott <sebott@xxxxxxxxxx>
Closes: https://lore.kernel.org/lkml/5d49767a-fbdc-fbe7-5fb2-d99ece3168cb@xxxxxxxxxx/
Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Thomas Weißschuh <linux@xxxxxxxxxxxxxx>
---

I'm not really familiar with the ELF loading process, so putting this
out as RFC.

A example binary compiled with aarch64-linux-gnu-gcc 13.2.0 is available
at https://test.t-8ch.de/binfmt-bss-repro.bin
---
 fs/binfmt_elf.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
index 7b3d2d491407..4008a57d388b 100644
--- a/fs/binfmt_elf.c
+++ b/fs/binfmt_elf.c
@@ -112,7 +112,7 @@ static struct linux_binfmt elf_format = {
 
 static int set_brk(unsigned long start, unsigned long end, int prot)
 {
-	start = ELF_PAGEALIGN(start);
+	start = ELF_PAGESTART(start);
 	end = ELF_PAGEALIGN(end);
 	if (end > start) {
 		/*

---
base-commit: aed8aee11130a954356200afa3f1b8753e8a9482
change-id: 20230914-bss-alloc-f523fa61718c

Best regards,
-- 
Thomas Weißschuh <linux@xxxxxxxxxxxxxx>