Re: [PATCH] RISC-V: Allow long jumps to __syscall_error

[Andrew Waterman <andrew@sifive.com>]

Use the `tail` pseudoinstruction rather than `la` + `jr`, as this will
relax to `j` when within range.


On Fri, Sep 16, 2022 at 6:31 AM Łukasz Stelmach via Libc-alpha
<libc-alpha@sourceware.org> wrote:
>
> __syscall_error may end up farther than 1MiB away from a caller,
> especially when linking statically large binaries. la+jr allow for
> 4GiB jumps.
>
> Fixes: 36960f0c76 ("RISC-V: Linux Syscall Interface")
> Fixes: 7f33b09c65 ("RISC-V: Linux ABI")
> Signed-off-by: Łukasz Stelmach <l.stelmach@samsung.com>
> ---
>
> I was compiling and statically linking GNU Guile and I was getting a
> lot of messages like this:
>
>     /usr/lib64/libc.a(uname.o): in function `__uname':
>     (.text+0x10): relocation truncated to fit: R_RISCV_JAL against symbol `__syscall_error' defined in .text section in /usr/lib64/libc.a(sysdep.o)
>
> And indeed in uname.o there was a relative jump which was marked as
> R_RISCV_JAL and pointed to __syscall_error. After sucessfully linking
> the binary with this fix applied __uname and __syscall_error end up
> 1125000 bytes apart.
>
> --8<---------------cut here---------------end--------------->8---
> Relocation section '.rela.text' at offset 0x250 contains 4 entries:
>   Offset          Info           Type           Sym. Value    Sym. Name + Addend
> 000000000000  00000000002b R_RISCV_ALIGN                        2
> 000000000002  00000000002b R_RISCV_ALIGN                        2
> 00000000000e  000a00000010 R_RISCV_BRANCH    0000000000000014 .Lsyscall_error__uname + 0
> 000000000014  000c00000011 R_RISCV_JAL       0000000000000000 __syscall_error + 0
> --8<---------------cut here---------------end--------------->8---
>
> --8<---------------cut here---------------end--------------->8---
> uname.o:     file format elf64-littleriscv
>
>
> Disassembly of section .text:
>
> 0000000000000000 <$x>:
>    0:   0001                    nop
>    2:   0001                    nop
>
> 0000000000000004 <__uname>:
>    4:   0a000893                li      a7,160
>    8:   00000073                ecall
>    c:   78fd                    lui     a7,0xfffff
>    e:   00a8e363                bltu    a7,a0,14 <.Lsyscall_error__uname>
>   12:   8082                    ret
>
> 0000000000000014 <.Lsyscall_error__uname>:
>   14:   fedff06f                j       0 <$x>
>  ^^^^                           ^^^^^^^^^^^^^^
>   18:   8082                    ret
>         ...
> --8<---------------cut here---------------end--------------->8---
>
> I suspect the two nops that the jump points to were meant(?)/could(??) be
> used (if there were more of them) to be a placeholder for a longer
> jump. Alas binutils 2.38 can't handle it properly (yet?), so a manual
> intervention here seems reasonable.
>
> I have neither seen any errors regarding code in *context.S and vfork.S
> files nor tested if the altered code works, but I assume it needs to be
> fixed as well.
>
>  sysdeps/unix/sysv/linux/riscv/clone.S       | 3 ++-
>  sysdeps/unix/sysv/linux/riscv/getcontext.S  | 3 ++-
>  sysdeps/unix/sysv/linux/riscv/setcontext.S  | 3 ++-
>  sysdeps/unix/sysv/linux/riscv/swapcontext.S | 3 ++-
>  sysdeps/unix/sysv/linux/riscv/sysdep.h      | 3 ++-
>  sysdeps/unix/sysv/linux/riscv/vfork.S       | 3 ++-
>  6 files changed, 12 insertions(+), 6 deletions(-)
>
> diff --git a/sysdeps/unix/sysv/linux/riscv/clone.S b/sysdeps/unix/sysv/linux/riscv/clone.S
> index d7d2915e87..e716285afa 100644
> --- a/sysdeps/unix/sysv/linux/riscv/clone.S
> +++ b/sysdeps/unix/sysv/linux/riscv/clone.S
> @@ -63,7 +63,8 @@ L (invalid):
>         li              a0, -EINVAL
>         /* Something bad happened -- no child created.  */
>  L (error):
> -       j               __syscall_error
> +       la              t0, __syscall_error
> +       jr              t0
>         END (__clone)
>
>  /* Load up the arguments to the function.  Put this block of code in
> diff --git a/sysdeps/unix/sysv/linux/riscv/getcontext.S b/sysdeps/unix/sysv/linux/riscv/getcontext.S
> index 499f70b65d..a4751ae595 100644
> --- a/sysdeps/unix/sysv/linux/riscv/getcontext.S
> +++ b/sysdeps/unix/sysv/linux/riscv/getcontext.S
> @@ -70,7 +70,8 @@ LEAF (__getcontext)
>
>         ret
>
> -99:    j       __syscall_error
> +99:    la      t0, __syscall_error
> +       jr      t0
>
>  PSEUDO_END (__getcontext)
>
> diff --git a/sysdeps/unix/sysv/linux/riscv/setcontext.S b/sysdeps/unix/sysv/linux/riscv/setcontext.S
> index e3bc84a2e6..fddd9d843f 100644
> --- a/sysdeps/unix/sysv/linux/riscv/setcontext.S
> +++ b/sysdeps/unix/sysv/linux/riscv/setcontext.S
> @@ -92,7 +92,8 @@ LEAF (__setcontext)
>
>         jr      t1
>
> -99:    j       __syscall_error
> +99:    la      t0, __syscall_error
> +       jr      t0
>
>  END (__setcontext)
>  libc_hidden_def (__setcontext)
> diff --git a/sysdeps/unix/sysv/linux/riscv/swapcontext.S b/sysdeps/unix/sysv/linux/riscv/swapcontext.S
> index 4da615f6d4..8e29d96595 100644
> --- a/sysdeps/unix/sysv/linux/riscv/swapcontext.S
> +++ b/sysdeps/unix/sysv/linux/riscv/swapcontext.S
> @@ -118,7 +118,8 @@ LEAF (__swapcontext)
>         jr      t1
>
>
> -99:    j       __syscall_error
> +99:    la      t0, __syscall_error
> +       jr      t0
>
>  PSEUDO_END (__swapcontext)
>
> diff --git a/sysdeps/unix/sysv/linux/riscv/sysdep.h b/sysdeps/unix/sysv/linux/riscv/sysdep.h
> index 37ff07a0d7..0cd78daf2b 100644
> --- a/sysdeps/unix/sysv/linux/riscv/sysdep.h
> +++ b/sysdeps/unix/sysv/linux/riscv/sysdep.h
> @@ -102,7 +102,8 @@
>  # else
>  #  define SYSCALL_ERROR_HANDLER(name)                          \
>  .Lsyscall_error ## name:                                       \
> -        j       __syscall_error;
> +        la t0, __syscall_error;                                        \
> +        jr t0;
>  # endif
>
>  /* Performs a system call, not setting errno.  */
> diff --git a/sysdeps/unix/sysv/linux/riscv/vfork.S b/sysdeps/unix/sysv/linux/riscv/vfork.S
> index 0970543619..8f1b2c9b3a 100644
> --- a/sysdeps/unix/sysv/linux/riscv/vfork.S
> +++ b/sysdeps/unix/sysv/linux/riscv/vfork.S
> @@ -39,7 +39,8 @@ LEAF (__libc_vfork)
>         bltz    a0, 1f
>         ret
>
> -1:     j               __syscall_error
> +1:     la      t0, __syscall_error
> +       jr      t0
>  END (__libc_vfork)
>
>  weak_alias (__libc_vfork, vfork)
> --
> 2.30.2
>