Hej,
since we figured that the proposed patch is not going to work I've spent a
couple more hours looking at this (some static binaries on arm64 segfault
during load [0]). The segfault happens because of a failed clear_user()
call in load_elf_binary(). The address we try to write zeros to is mapped with
correct permissions.
After some experiments I've noticed that writing to anonymous mappings work
fine and all the error cases happend on file backed VMAs. Debugging showed that
in elf_map() we call vm_mmap() with a file offset of 15 pages - for a binary
that's less than 1KiB in size.
Looking at the ELF headers again that 15 pages offset originates from the offset
of the 2nd segment - so, I guess the loader did as instructed and that binary is
just too nasty?
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
As an additional test I've added a bunch of zeros at the end of that binary
so that the offset is within that file and it did load just fine.
On the other hand there is this section header:
[ 4] .bss NOBITS 000000000041ffe8 0000ffe8
0000000000000008 0000000000000000 WA 0 0 1
"sh_offset
This member's value gives the byte offset from the beginning of the file to
the first byte in the section. One section type, SHT_NOBITS described
below, occupies no space in the file, and its sh_offset member locates
the conceptual placement in the file.
"
So, still not sure what to do here..
Sebastian
[0] https://lore.kernel.org/lkml/5d49767a-fbdc-fbe7-5fb2-d99ece3168cb@xxxxxxxxxx/
