linux-kernel - Re: [PATCH v2] x86/mm: Fix RESERVE

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Date:   Thu, 9 Jun 2022 11:03:33 +0300
From:   Joe Damato <jdamato@...tly.com>
To:     Josh Poimboeuf <jpoimboe@...nel.org>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Andrew Cooper <Andrew.Cooper3@...rix.com>,
        jiangshanlai@...il.com, bp@...e.de, brgerst@...il.com,
        Byungchul Park <byungchul.park@....com>
Subject: Re: [PATCH v2] x86/mm: Fix RESERVE_BRK() for older binutils

On Thu, Jun 09, 2022 at 12:17:32AM -0700, Josh Poimboeuf wrote:
> With binutils 2.26, RESERVE_BRK() causes a build failure:
>
>   /tmp/ccnGOKZ5.s: Assembler messages:
>   /tmp/ccnGOKZ5.s:98: Error: missing ')'
>   /tmp/ccnGOKZ5.s:98: Error: missing ')'
>   /tmp/ccnGOKZ5.s:98: Error: missing ')'
>   /tmp/ccnGOKZ5.s:98: Error: junk at end of line, first unrecognized
>   character is `U'
>
> The problem is this line:
>
>   RESERVE_BRK(early_pgt_alloc, INIT_PGT_BUF_SIZE)
>
> Specifically, the INIT_PGT_BUF_SIZE macro which (via PAGE_SIZE's use
> _AC()) has a "1UL", which makes older versions of the assembler unhappy.
> Unfortunately the _AC() macro doesn't work for inline asm.
>
> Inline asm was only needed here to convince the toolchain to add the
> STT_NOBITS flag.  However, if a C variable is placed in a section whose
> name is prefixed with ".bss", GCC and Clang automatically set
> STT_NOBITS.  In fact, ".bss..page_aligned" already relies on this trick.
>
> So fix the build failure (and simplify the macro) by allocating the
> variable in C.
>
> Also, add NOLOAD to the ".brk" output section clause in the linker
> script.  This is a failsafe in case the ".bss" prefix magic trick ever
> stops working somehow.  If there's a section type mismatch, the GNU
> linker will force the ".brk" output section to be STT_NOBITS.  The LLVM
> linker will fail with a "section type mismatch" error.
>
> Note this also changes the name of the variable from .brk.##name to
> __brk_##name.  The variable names aren't actually used anywhere, so it's
> harmless.
>
> Reported-by: Joe Damato <jdamato@...tly.com>
> Reported-by: Byungchul Park <byungchul.park@....com>
> Fixes: a1e2c031ec39 ("x86/mm: Simplify RESERVE_BRK()")
> Signed-off-by: Josh Poimboeuf <jpoimboe@...nel.org>
> ---
>  arch/x86/include/asm/setup.h  | 38 +++++++++++++++++++----------------
>  arch/x86/kernel/setup.c       |  5 -----
>  arch/x86/kernel/vmlinux.lds.S |  4 ++--
>  3 files changed, 23 insertions(+), 24 deletions(-)
>
> diff --git a/arch/x86/include/asm/setup.h b/arch/x86/include/asm/setup.h
> index 7590ac2570b9..f8b9ee97a891 100644
> --- a/arch/x86/include/asm/setup.h
> +++ b/arch/x86/include/asm/setup.h
> @@ -108,19 +108,16 @@ extern unsigned long _brk_end;
>  void *extend_brk(size_t size, size_t align);
>
>  /*
> - * Reserve space in the brk section.  The name must be unique within the file,
> - * and somewhat descriptive.  The size is in bytes.
> + * Reserve space in the .brk section, which is a block of memory from which the
> + * caller is allowed to allocate very early (before even memblock is available)
> + * by calling extend_brk().  All allocated memory will be eventually converted
> + * to memblock.  Any leftover unallocated memory will be freed.
>   *
> - * The allocation is done using inline asm (rather than using a section
> - * attribute on a normal variable) in order to allow the use of @nobits, so
> - * that it doesn't take up any space in the vmlinux file.
> + * The size is in bytes.
>   */
> -#define RESERVE_BRK(name, size)                                              \
> -     asm(".pushsection .brk_reservation,\"aw\",@nobits\n\t"          \
> -         ".brk." #name ":\n\t"                                       \
> -         ".skip " __stringify(size) "\n\t"                           \
> -         ".size .brk." #name ", " __stringify(size) "\n\t"           \
> -         ".popsection\n\t")
> +#define RESERVE_BRK(name, size)                                      \
> +     __section(".bss..brk") __aligned(1) __used      \
> +     static char __brk_##name[size]
>
>  extern void probe_roms(void);
>  #ifdef __i386__
> @@ -133,12 +130,19 @@ asmlinkage void __init x86_64_start_reservations(char *real_mode_data);
>
>  #endif /* __i386__ */
>  #endif /* _SETUP */
> -#else
> -#define RESERVE_BRK(name,sz)                         \
> -     .pushsection .brk_reservation,"aw",@nobits;     \
> -.brk.name:                                           \
> -1:   .skip sz;                                       \
> -     .size .brk.name,.-1b;                           \
> +
> +#else  /* __ASSEMBLY */
> +
> +.macro __RESERVE_BRK name, size
> +     .pushsection .bss..brk, "aw"
> +SYM_DATA_START(__brk_\name)
> +     .skip \size
> +SYM_DATA_END(__brk_\name)
>       .popsection
> +.endm
> +
> +#define RESERVE_BRK(name, size) __RESERVE_BRK name, size
> +
>  #endif /* __ASSEMBLY__ */
> +
>  #endif /* _ASM_X86_SETUP_H */
> diff --git a/arch/x86/kernel/setup.c b/arch/x86/kernel/setup.c
> index 3ebb85327edb..bd6c6fd373ae 100644
> --- a/arch/x86/kernel/setup.c
> +++ b/arch/x86/kernel/setup.c
> @@ -67,11 +67,6 @@ RESERVE_BRK(dmi_alloc, 65536);
>  #endif
>
>
> -/*
> - * Range of the BSS area. The size of the BSS area is determined
> - * at link time, with RESERVE_BRK() facility reserving additional
> - * chunks.
> - */
>  unsigned long _brk_start = (unsigned long)__brk_base;
>  unsigned long _brk_end   = (unsigned long)__brk_base;
>
> diff --git a/arch/x86/kernel/vmlinux.lds.S b/arch/x86/kernel/vmlinux.lds.S
> index f5f6dc2e8007..81aba718ecd5 100644
> --- a/arch/x86/kernel/vmlinux.lds.S
> +++ b/arch/x86/kernel/vmlinux.lds.S
> @@ -385,10 +385,10 @@ SECTIONS
>       __end_of_kernel_reserve = .;
>
>       . = ALIGN(PAGE_SIZE);
> -     .brk : AT(ADDR(.brk) - LOAD_OFFSET) {
> +     .brk (NOLOAD) : AT(ADDR(.brk) - LOAD_OFFSET) {
>               __brk_base = .;
>               . += 64 * 1024;         /* 64k alignment slop space */
> -             *(.brk_reservation)     /* areas brk users have reserved */
> +             *(.bss..brk)            /* areas brk users have reserved */
>               __brk_limit = .;
>       }
>
> --
> 2.34.3
>

I applied the v2 patch on top of commit 58f9d52ff689 ("Merge tag
'net-5.19-rc1' of
git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net") and the kernel
builds successfully for me.

The resulting kernel boots fine on the machine, as well.

I built the kernel with:
  - gcc (Ubuntu 5.4.0-6ubuntu1~16.04.12) 5.4.0 20160609
  - binutils 2.26.1-1ubuntu1~16.04.8+esm1

I have not tested with any other build toolchains.

Tested-by: Joe Damato <jdamato@...tly.com>