Whacky ia64: linux_proc_xfer_partial and lseek vs pread64

[David Lecomber <david@lecomber.net>]

Dear Dan and all,

It's great that these days we use file access to get at the memory via
the /proc filesystem - but there's an interesting sighting on the ia64
(suse 9) in linux_proc_xfer_partial.

#ifdef HAVE_PREAD64
  if (pread64 (fd, readbuf, len, offset) != len)
#else
  if (lseek (fd, offset, SEEK_SET) == -1 || read (fd, readbuf, len) !=
len)
#endif
    ret = 0;
  else
    ret = len;


So, Mr Itanium has pread64, it calls pread64..  it seems to fail
regularly..  As the strace log shows.

rt_sigprocmask(SIG_BLOCK, NULL, [], 8)  = 0
open("/proc/21785/mem", O_RDONLY)       = 4
pread(4, 0x60000fffffffa040, 64, 11529215046068469760) = -1 EINVAL
(Invalid argument)
close(4)                                = 0
ptrace(PTRACE_PEEKTEXT, 21785, 0xa000000000000000, NULL) =
282584257676671
open("/proc/21785/mem", O_RDONLY)       = 4
pread(4, 0x60000fffffffa048, 56, 11529215046068469768) = -1 EINVAL
(Invalid argument)
close(4)                                = 0
ptrace(PTRACE_PEEKTEXT, 21785, 0xa000000000000008, NULL) = 0
open("/proc/21785/mem", O_RDONLY)       = 4


But, if you change this to actually call lseek, instead of pread, it's a
bit more successful.  

rt_sigprocmask(SIG_BLOCK, NULL, [], 8)  = 0
open("/proc/22498/mem", O_RDONLY)       = 4
lseek(4, 11529215046068469760, SEEK_SET) = 11529215046068469760
read(4, "\177ELF\2\1\1\0\0\0\0\0\0\0\0\0\3\0002\0\1\0\0\0`\6\1\0"...,
64) = 64
close(4)                                = 0
open("/proc/22498/mem", O_RDONLY)       = 4
lseek(4, 11529215046068469824, SEEK_SET) = 11529215046068469824
read(4, "\1\0\0\0\4\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\240\0\0"...,
224) = 224
close(4)                                = 0
open("/proc/22498/mem", O_RDONLY)       = 4
lseek(4, 11529215046068469760, SEEK_SET) = 11529215046068469760
read(4, "\177ELF\2\1\1\0\0\0\0\0\0\0\0\0\3\0002\0\1\0\0\0`\6\1\0"...,
3408) = 3408

Literally saving over 500 calls to ptrace() in just one stroke.

Anyone any idea what's going on?  I'd be happy to let someone else
formulate the rather obvious patch, as I don't know the behaviour on
other platforms.

d.
-- 
David Lecomber <david@lecomber.net>