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Re: [binutils-gdb] Fix the linker so that it will not silently generate ELF binaries with invalid program headers. Fix
- From: "Maciej W. Rozycki" <macro at imgtec dot com>
- To: Alan Modra <amodra at gmail dot com>
- Cc: Nick Clifton <nickc at sourceware dot org>, <binutils at sourceware dot org>
- Date: Fri, 9 Dec 2016 00:17:31 +0000
- Subject: Re: [binutils-gdb] Fix the linker so that it will not silently generate ELF binaries with invalid program headers. Fix
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- References: <email@example.com> <alpine.DEB.firstname.lastname@example.org> <20161208113238.GF10584@bubble.grove.modra.org>
On Thu, 8 Dec 2016, Alan Modra wrote:
> > Program Headers:
> > Type Offset VirtAddr PhysAddr FileSiz MemSiz Flg Align
> > PHDR 0x000034 0x00001034 0x00000000 0x000a0 0x000a0 R E 0x4
> > INTERP 0x001000 0x00080000 0x00080000 0x00013 0x00013 R 0x1
> > [Requesting program interpreter: /usr/lib/libc.so.1]
> > LOAD 0x001000 0x00080000 0x00080000 0x00408 0x00408 R E 0x1000
> > LOAD 0x002000 0x00081000 0x00081000 0x00804 0x00c00 RW 0x1000
> > DYNAMIC 0x002000 0x00081000 0x00081000 0x00078 0x00078 RW 0x4
> The gABI says:
> The array element, if present, specifies the location and size of
> the program header table itself, both in the file and in the
> memory image of the program. This segment type may not occur more
> than once in a file. Moreover, it may occur only if the program
> header table is part of the memory image of the program. If it is
> present, it must precede any loadable segment entry.
> The above clearly violates this part of the spec because PT_PHDR is
> present yet is not part of the memory image.
Well, TBH I think it's all but clear, given that nowhere in the ELF gABI
that I can find there is an actual definition of what "the memory image"
It might be possible to prove it by contradiction from the "Base Address"
section, where it is specified that it is `p_vaddr' of the lowest PT_LOAD
segment that is used to calculate the base address or the relocation
amount for the whole image, that no other segment type is a part of the
memory image without a corresponding PT_LOAD shadow. But I wouldn't be
that convinced about it as arguably the definition does not actually
*require* that there is no mapped area below the lowest PT_LOAD segment --
it merely specifies the calculation algorithm which needs a point of
reference, and which may well apply to any other segment types, even if
mapped below the lowest PT_LOAD segment.
It may also be important to note that based on the change bars in the
latest 4.1 PDF version of the gABI it can be inferred that the "Base
Address" section was updated relatively late in the course of the
document's processing and it cannot be completely ruled out that parts
elsewhere were missed out and not revised accordingly.
Contrariwise, it is specified that `p_memsz' gives "the number of bytes
in the memory image of the segment" without limiting it to PT_LOAD
segments, so one could argue all segments whose this value is non-zero
comprise the memory image, regardless of whether they are shadowed by a
PT_LOAD segment or not.
Intuitively I think that you are correct in that it was the intent of the
gABI authors that only PT_LOAD segments comprise the memory image, but I
also think that intuition and guessing the intent is not the right way of
interpreting a technical specification, as intuition varies among people
and some implementers may have understood the document differently and
still be formally right in their interpretation.
> Nick's patch forced the first PT_LOAD to cover the program headers. I
> think an equally valid and somewhat better fix would have been to not
> emit PT_PHDR when no PT_LOAD header covers the program headers. The
> reason I say that is because PT_PHDR is optional. A loader can read
> the program headers itself from file using info in the ELF header.
There may be no loader available to load the program headers or the ELF
file header if a bare-metal ELF image has been externally loaded according
to some interpretation of the headers, and the binary wants to access its
own structures at run time. Existing environments may rely on our current
semantics even if it is not strictly compliant.
I think my inability to actually decide which way is correct for MIPS
VxWorks (for which I have no environment available to verify) just shows
that changing semantics which we had since forever just to match a piece
of (virtual) paper from 20 years ago may not be the greatest idea, and
certainly needs a lot of care and consideration if it is given a go-ahead.
There may well be environments out there which have relied on our
technically broken semantics for an equally long period of time and we may
well have thus established the de-facto standard. There may be parts of
the environments we have no control over and which may be difficult to
correct to match the written standard.
I mean if there is a case of actual breakage that our implementation
causes, as it has been in this case, then we certainly do ought to address
it, one way or another. So we found a way to fix Linux, but are we
confident it didn't break anything else? Maybe we need a different fix.
Or at least a linker script or command-line option to chicken out and
resort to the old semantics?
But perhaps I'm just being overly cautious. So is anyone out there who
could actually regression-test say GCC with current binutils on MIPS
VxWorks, or run whatever is usually done for toolchain validation with
that target? After all what matters is whether the environment continues
operating correctly and not whether a cryptic test case from 10 years ago
passes unchanged or not. It might be that the test case is too strict or
genuinely wrong in some way.