[Bug tapsets/16058] New: syscall.mmap/nd_syscall.mmap probe alias problem

["dsmith at redhat dot com" <sourceware-bugzilla at sourceware dot org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=16058

            Bug ID: 16058
           Summary: syscall.mmap/nd_syscall.mmap probe alias problem
           Product: systemtap
           Version: unspecified
            Status: NEW
          Severity: normal
          Priority: P2
         Component: tapsets
          Assignee: systemtap at sourceware dot org
          Reporter: dsmith at redhat dot com

Bug #11388 pointed out a problem with syscall.mmap* probes on 2.6.33+ kernels.
The solutions was to switch to probing sys_mmap_pgoff() for those kernels.

I've found a bug in this transition. The last argument to mmap() is supposed to
be an offset in the file, and must be a multiple of the page size. But, for
mmap_pgoff() that offset is the number of pages.

So:

  mmap(..., 8192)

is equivalent to:

  mmap_pgoff(..., 2)

when the page size is 4k.

In the new syscall.mmap/nd_syscall.mmap probe alias using sys_mmap_pgoff(),
this difference isn't taken into account, and we're returning 2 instead of
8192.

This wasn't noticed since we don't have a test that calls mmap() with a
non-zero offset.

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