[Bug binutils/30623] New: objcopy --set-section-flags: support toggling a flag

[i at maskray dot me]

https://sourceware.org/bugzilla/show_bug.cgi?id=30623

            Bug ID: 30623
           Summary: objcopy --set-section-flags: support toggling a flag
           Product: binutils
           Version: unspecified
            Status: NEW
          Severity: normal
          Priority: P2
         Component: binutils
          Assignee: unassigned at sourceware dot org
          Reporter: i at maskray dot me
  Target Milestone: ---

It will be more convenient if --set-section-flags supports toggling a flag,
instead of specifying the full set of flags, e.g.

--set-section-flags .foo=-alloc
--set-section-flags .foo=-readonly
--set-section-flags .foo=+readonly

The lack of toggling flags is currently not so cumbersome because most ELF
sections only use "alloc" and "readonly".
However, if we consider more flags, e.g. "exclude", "large"
(https://sourceware.org/bugzilla/show_bug.cgi?id=30592), the lack of toggling
flags is quite inconvenient.
For example, if we want to add SHF_X86_64_LARGE to large data sections, we need
to figure out whether "readonly" is set.

--set-section-flags .ldata=alloc,large
--set-section-flags .lrodata=alloc,readonly,large

It will be more convenient if we can just use

--set-section-flags .ldata=+large
--set-section-flags .lrodata=+large

Candidate syntax: the right hand side of `=` can use one of the two forms:

* comma separated flags, e.g. alloc,large
* comma separated + flags and - flags, e.g. -alloc,+readonly

Other forms like -alloc,readonly probably should be rejected.

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