[PATCH] D67689: [llvm-objcopy] Add support for --gap-fill and --pad-to options

[Jake Ehrlich via Phabricator via llvm-commits]

jakehehrlich added a comment.

"Interstitial data" is data covered by by a segment but not a section. Gap fill should IMO set the bytes for all of that space. This is consistent with James's recommendation how things work. --pad-to when padding to a page aligned address this can for instance ensure that all executable bytes contain either valid code or a trap instruction.

Another axiom I'd like to hold if possible for the use case: no new segment overlapping should occur and its an error if --pad-to causes this. Technically this is valid and can occur and the internals handle it so I'm not adding a hard no here.

Dear god lets throw an error for now if this has to touch a TLS segment/sections...thats a crazy rabit whole where the rules for how everything works change. When someone hits that case we can think about it then.

If the last PT_LOAD segment memsize and filesize don't agree then using -pad-to is invalid and should be be an error.

I'm still not satisfied I've thought of all the error conditions so expect more as I think about this more carefully.



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Comment at: llvm/tools/llvm-objcopy/ObjcopyOpts.td:229
+    : Eq<"gap-fill",
+      "Fill gaps between two loadable sections with <val>.">,
+      MetaVarName<"val">;
----------------
I'd say "set all bytes not covered by an allocated section but covered by a PT_LOAD segment to <val>" or something explicit to that effect. Reading GNU objcopy's format agnostic documentation is far too vauge to guess the results of.


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Comment at: llvm/tools/llvm-objcopy/ObjcopyOpts.td:234
+    : Eq<"pad-to",
+      "Pad the last loadable section up to <addr> with the value specified by --gap-fill if present or zero by default.">,
+      MetaVarName<"addr">;
----------------
jhenderson wrote:
> This line is too long and should be broken up over multiple lines, like the other switch names. I think there's a clang-format invocation to do that for tablegen files, but I'm not sure what it is.
We should be explict and say "pad the last SHF_ALLOC section ...."


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  https://reviews.llvm.org/D67689/new/

https://reviews.llvm.org/D67689