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doc: Add plain text readme for using GCS
TODO: this is just for the arm/gcs branch while it is being developed
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README
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README
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this branch contains experimental GCS support (not ABI stable)
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source and branches
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-------------------
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binutils-gdb: upstream-git users/ARM/gcs-binutils-gdb-master
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gcc (trunk): upstream-git vendors/ARM/gcs
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gcc (gcc-13): upstream-git vendors/ARM/gcs-13
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note: gcc vendor branches need setup https://gcc.gnu.org/gitwrite.html#vendor
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glibc: upstream-git arm/gcs
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linux: https://git.kernel.org/pub/scm/linux/kernel/git/broonie/misc.git arm64-gcs
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fvp fast model can be used for testing.
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toolchain build
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---------------
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two options:
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(1) branch-protect by default
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configure gcc with --enable-standard-branch-protection
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and build glibc normally
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(2) do not branch-protect by default, require explicit cflags
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configure gcc with
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CFLAGS_FOR_TARGET='-O2 -mbranch-protection=standard'
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CXXFLAGS_FOR_TARGET='-O2 -mbranch-protection=standard'
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and configure glibc with
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CFLAGS='-g -O2 -mbranch-protection=standard'
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build user code with
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CFLAGS+=-mbranch-protection=standard
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(equivalent to -mbranch-protection=bti+pac+gcs)
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linking
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-------
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use ldflags:
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-z experimental-gcs={always,never,implicit}
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always: force GCS marking on
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never: force GCS marking off
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implicit: mark output if all inputs are marked (default)
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-z experimental-gcs-report={none,warning,error}
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none: silent (default)
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warning: when output is marked, unmarked input is a warning
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error: when output is marked, unmarked input is an error
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runtime
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-------
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run with environment var
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GLIBC_TUNABLES=glibc.cpu.aarch64_gcs=1:glibc.cpu.aarch64_gcs_policy=2
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by default both tunables are 0, the meaning is
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glibc.cpu.aarch64_gcs_policy=0:
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GCS is enabled if glibc.cpu.aarch64_gcs is set
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glibc.cpu.aarch64_gcs_policy=1:
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GCS is enabled if glibc.cpu.aarch64_gcs is set and binary is marked
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if GCS is enabled an incompatible dlopen is an error
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glibc.cpu.aarch64_gcs_policy=2:
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GCS is enabled if glibc.cpu.aarch64_gcs is set
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if GCS is enabled any incompatible binary is an error
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original readme
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---------------
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This directory contains the sources of the GNU C Library.
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See the file "version.h" for what release version you have.
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