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    SubjectRe: [PATCH] tools: bpf: Disable stack protector
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    >> On 1/16/23 2:49 PM, Peter Foley wrote:
    >>> On Mon, Jan 16, 2023 at 4:59 AM Eduard Zingerman <eddyz87@gmail.com> wrote:
    >>>>
    >>>> A bit tangential, but since BPF LLVM backend does not support the
    >>>> stack protector (should it?) there is also an option to adjust LLVM
    >>>> to avoid this instrumentation, WDYT?
    >>>>
    >>> That would probably be worth doing, yes.
    >>> But given that won't help already released versions of clang, it
    >>> should probably happen in addition to this patch.
    >>
    >> Peter,
    >>
    >> If I understand correctly (by inspecting clang code), the stack
    >> protector is off by default. Do you have link to Gentoo build
    >> page to show how they enable stack protector? cmake config or
    >> a private patch?
    >>
    >> Jose,
    >>
    >> How gcc-bpf handle stack protector? The compiler just disables
    >> stack protector for bpf target?
    >
    > It doesn't. -fstack-protector is disabled by default in GCC. When you
    > use it you get something like:
    >
    > $ echo 'int foo() { char s[256]; return s[3]; }' | bpf-unknown-none-gcc \
    > -fstack-protector -S -o foo.s -O2 -xc -
    > $ cat foo.s
    > .file "<stdin>"
    > .text
    > .align 3
    > .global foo
    > .type foo, @function
    > foo:
    > lddw %r1,__stack_chk_guard
    > ldxdw %r0,[%r1+0]
    > stxdw [%fp+-8],%r0
    > ldxb %r0,[%fp+-261]
    > lsh %r0,56
    > arsh %r0,56
    > ldxdw %r2,[%fp+-8]
    > ldxdw %r3,[%r1+0]
    > jne %r2,%r3,.L4
    > exit
    > .L4:
    > call __stack_chk_fail
    > .size foo, .-foo
    > .ident "GCC: (GNU) 12.0.0 20211206 (experimental)"
    >
    > i.e. it pushes a stack canary and checks it upon function exit, calling
    > __stack_chk_fail.
    >
    > If clang has -fstack-protector ON by default and you change the BPF
    > backend in order to ignore the flag, I think we should do the same in
    > GCC.

    I went ahead and pushed the patch below to GCC master. If
    -fstack-protector is ever considered useful in the architecture, we can
    always stop disabling it.

    I would recommend to change the default for -fstack-protector in clang
    to be off by default when targetting BPF targets, and to emit the same
    or similar note to the user when the option is enabled explicitly with
    -fstack-protector:

    note: ‘-fstack-protector’ does not work on this architecture

    WDYT?

    From 3b81f5c4d8e0d79cbd6927d004185707c14e54b2 Mon Sep 17 00:00:00 2001
    Date: Tue, 17 Jan 2023 17:16:32 +0100
    Subject: [COMMITTED] bpf: disable -fstack-protector in BPF

    The stack protector is not supported in BPF. This patch disables
    -fstack-protector in bpf-* targets, along with the emission of a note
    indicating that the feature is not supported in this platform.

    Regtested in bpf-unknown-none.

    gcc/ChangeLog:

    * config/bpf/bpf.cc (bpf_option_override): Disable
    -fstack-protector.
    ---
    gcc/config/bpf/bpf.cc | 8 ++++++++
    1 file changed, 8 insertions(+)

    diff --git a/gcc/config/bpf/bpf.cc b/gcc/config/bpf/bpf.cc
    index 576a1fe8eab..b268801d00c 100644
    --- a/gcc/config/bpf/bpf.cc
    +++ b/gcc/config/bpf/bpf.cc
    @@ -253,6 +253,14 @@ bpf_option_override (void)
    if (bpf_has_jmp32 == -1)
    bpf_has_jmp32 = (bpf_isa >= ISA_V3);

    + /* Disable -fstack-protector as it is not supported in BPF. */
    + if (flag_stack_protect)
    + {
    + inform (input_location,
    + "%<-fstack-protector%> does not work "
    + " on this architecture");
    + flag_stack_protect = 0;
    + }
    }

    #undef TARGET_OPTION_OVERRIDE
    --
    2.30.2
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