• Alexey Dobriyan's avatar
    cred: simpler, 1D supplementary groups · 81243eac
    Alexey Dobriyan authored
    Current supplementary groups code can massively overallocate memory and
    is implemented in a way so that access to individual gid is done via 2D
    array.
    
    If number of gids is <= 32, memory allocation is more or less tolerable
    (140/148 bytes).  But if it is not, code allocates full page (!)
    regardless and, what's even more fun, doesn't reuse small 32-entry
    array.
    
    2D array means dependent shifts, loads and LEAs without possibility to
    optimize them (gid is never known at compile time).
    
    All of the above is unnecessary.  Switch to the usual
    trailing-zero-len-array scheme.  Memory is allocated with
    kmalloc/vmalloc() and only as much as needed.  Accesses become simpler
    (LEA 8(gi,idx,4) or even without displacement).
    
    Maximum number of gids is 65536 which translates to 256KB+8 bytes.  I
    think kernel can handle such allocation.
    
    On my usual desktop system with whole 9 (nine) aux groups, struct
    group_info shrinks from 148 bytes to 44 bytes, yay!
    
    Nice side effects:
    
     - "gi->gid[i]" is shorter than "GROUP_AT(gi, i)", less typing,
    
     - fix little mess in net/ipv4/ping.c
       should have been using GROUP_AT macro but this point becomes moot,
    
     - aux group allocation is persistent and should be accounted as such.
    
    Link: http://lkml.kernel.org/r/20160817201927.GA2096@p183.telecom.bySigned-off-by: default avatarAlexey Dobriyan <adobriyan@gmail.com>
    Cc: Vasily Kulikov <segoon@openwall.com>
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    81243eac
uid16.c 4.95 KB