fs/binfmt_elf: use PT_LOAD p_align values for suitable start address

fs/binfmt_elf: use PT_LOAD p_align values for suitable start address

Patch series "Selecting Load Addresses According to p_align", v3.

The current ELF loading mechancism provides page-aligned mappings.  This
can lead to the program being loaded in a way unsuitable for file-backed,
transparent huge pages when handling PIE executables.

While specifying -z,max-page-size=0x200000 to the linker will generate
suitably aligned segments for huge pages on x86_64, the executable needs
to be loaded at a suitably aligned address as well.  This alignment
requires the binary's cooperation, as distinct segments need to be
appropriately paddded to be eligible for THP.

For binaries built with increased alignment, this limits the number of
bits usable for ASLR, but provides some randomization over using fixed
load addresses/non-PIE binaries.

This patch (of 2):

The current ELF loading mechancism provides page-aligned mappings.  This
can lead to the program being loaded in a way unsuitable for file-backed,
transparent huge pages when handling PIE executables.

For binaries built with increased alignment, this limits the number of
bits usable for ASLR, but provides some randomization over using fixed
load addresses/non-PIE binaries.

Tested by verifying program with -Wl,-z,max-page-size=0x200000 loading.

[[email protected]: fix max() warning]
[[email protected]: augment comment]
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Chris Kennelly <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Cc: Alexander Viro <[email protected]>
Cc: Alexey Dobriyan <[email protected]>
Cc: Song Liu <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: Ian Rogers <[email protected]>
Cc: Hugh Dickens <[email protected]>
Cc: Suren Baghdasaryan <[email protected]>
Cc: Sandeep Patil <[email protected]>
Cc: Fangrui Song <[email protected]>
Cc: Nick Desaulniers <[email protected]>
Cc: "Kirill A. Shutemov" <[email protected]>
Cc: Mike Kravetz <[email protected]>
Cc: Shuah Khan <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Linus Torvalds <[email protected]>