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[Bug binutils/25181] New: RISC-V: Linker relaxation may fail if there ar

From: yitingwang16 at outlook dot com
Subject: [Bug binutils/25181] New: RISC-V: Linker relaxation may fail if there are R_RISCV_ALIGN type relocations
Date: Sun, 10 Nov 2019 03:31:16 +0000


            Bug ID: 25181
           Summary: RISC-V: Linker relaxation may fail if there are
                    R_RISCV_ALIGN type relocations
           Product: binutils
           Version: 2.32
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: binutils
          Assignee: unassigned at sourceware dot org
          Reporter: yitingwang16 at outlook dot com
  Target Milestone: ---

If there are two callers in two .text sections (for example sections A, B),
followed by two sections with R_RISCV_ALIGN type relocations (section C, D),
then followed by the called functions in sections behind (section E), there is
a chance to reproduce the issue.

I draw a picture to illustrate the problem:

       high        |                                   |
        ^       -> | section E: cc, dd                 | <-
        |       |  |___________________________________|  |
        |       |  |                                   |  |
        |       |  | section D: align2 (R_RISCV_ALIGN) |  |
        |       |  |___________________________________|  |
        |       |  |                                   |  |
        |       |  | section C: align1 (R_RISCV_ALIGN) |  |
        |       |  |___________________________________|  |
        |       |  |                                   |  |
        |       |  | section B : ff  (R_RISCV_CALL)    |--
        |       |  |___________________________________|
        |       |  |                                   |
  address low   -- | section A : _start (R_RISCV_CALL) |

Consider this situation: Section A, B, C, D, E are linked in sequence. Section
C and D have R_RISCV_ALIGN type relocations. Function _start() calls function
cc(), function ff() calls dd(). The size of each section is 8, 8, 64, 0xfffbc,
10 bytes before relaxation.

In the first relaxation pass (info->relax_pass ==0), the R_RISCV_CALL
relocations in section A and B couldn't be relaxed to R_RISCV_JAL, because the
call from _start() to cc() and the call from ff() to dd() don't fit with 21-bit
offset. The offset is 0x10001c indeed. In the first relaxation pass, nothing is

In the third relaxation pass (info->relax_pass ==2), the R_RISCV_ALIGN
relocations in section C and D could be relaxed. The sizes of section C and D
were changed to 34 and 0xfffba bytes. Now, the offset of the call from _start()
to cc() is 0xffffe, the offset from ff() to dd() equals to this value too.

In the second round of relaxation, in the first pass (info->relax_pass ==0),
the R_RISCV_CALL relocation in section A could be relaxed to R_RISCV_JAL. After
the relaxation, section A's size is reduced by 4 bytes. So, section B's base
address and every symbol go down 4 bytes forward. However section C, D and E
will not go down 4 bytes, because of the .balign restriction placed in section
C and D.
But, when linker processes the relaxation of section B, it uses the original
section B symbol addresses to calculate the offset of R_RISCV_CALL relocation
(from ff() to dd()). The offset is within 1M bytes offset (0xffffe).  So, the
R_RISCV_CALL relocation is relaxed to R_RISCV_JAL. This is a mistake!
When linker finally performs the relocation (in perform_relocation()), the
section B's symbol has been adjusted, then the bug is exposed.

I created a simple test case to trigger this issue:

    riscv64-unknown-linux-gnu-gcc -nostdlib -o a.out a.s align_1.s align_2.s
    /tmp/ccgFgKhw.o: In function `ff':
    (.text+0x0): relocation truncated to fit: R_RISCV_JAL against symbol `dd'
defined in .text section in /tmp/ccR0Ha6r.o

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