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Re: bfd_mmap for archive members

On 11/18/2009 06:45 AM, Tristan Gingold wrote:
currently bfd_mmap does nothing particular with archive members, ie it doesn't add the member offset in the
archive to the offset argument.  This is not the behaviour of bfd_seek (which handles archive members).

I today hit this issue because gdb/dwarf2read.c uses bfd_mmap to read dwarf and because Darwin has fat
binaries, the bfd may be a archive member.

I don't think that we can fix bfd_mmap because gdb/dwarf2read.c does address arithmetic with the offset to
align it to a page.  Adding member offset would invalidate this calculation.  Maybe the interface of bfd_mmap
was not designed for member but I suppose it is too late to change it.

It depends on the purpose for which dwarf2read.c does the alignment, and what it expects of the mapped address. If the only reason for alignment is to make mmap() *possible* [that is, to choose (0xfff & memory_address) == (0xfff & file_offset)], and if dwarf2read.c does not care that the low-order 12 bits of the result might not be zero, and if the mapping is MAP_PRIVATE [changes to memory are not written back to the file], then it should be possible to interpret the request as "Please map an interval of the file into memory, so that this member is completely contained in the mapped interval, and return the address of the beginning of the member. I promise not to scribble on anything that matters." [It is conceivable that dwarf2read.c might currently "try too hard", and might need some adjustment to handle the implementation of bfd_mmap.]

I plan to post a path on gdb-patches to fix this issue but I'd like to have a confirmation that this is
the right way to do it.  If so I also plan to improve the documentation for bfd_mmap to make the handling
of archive members clearer.

These three threads in the last six months have 'bfd_mmap'. How does the proposal relate to them?

  How to get file descriptor from abfd?

  PATCH: Regeneated bfd-in2.h

  [RFC] Implement bfd_iovec for BFD_IN_MEMORY


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