8000 Use symcheck to locate writeable+executable object files by tgross35 · Pull Request #934 · rust-lang/compiler-builtins · GitHub
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Use symcheck to locate writeable+executable object files #934

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@tgross35 tgross35 commented May 30, 2025 •
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From what I have been able to find, compilers that try to emit object files compatible with a GNU linker appear to add a .note.GNU-stack section if the stack should not be writeable (this section is empty). We never want a writeable stack, so extend the object file check to verify that object files with any executable sections also have this .note.GNU-stack section.

This appears to match what is done by scanelf to emit !WX 1, which is the tool used to create the output in the issue.

Closes: #183
@tgross35
Use symcheck to locate writeable+executable object files
9f44c32 
From what I have been able to find, compilers that try to emit object
files compatible with a GNU linker appear to add a `.note.GNU-stack`
section if the stack should not be writeable (this section is empty). We
never want a writeable stack, so extend the object file check to verify
that object files with any executable sections also have this
`.note.GNU-stack` section.

This appears to match what is done by `scanelf` to emit `!WX` [2], which
is the tool used to create the output in the issue.

Closes: rust-lang#183

[1]: https://github.com/gentoo/pax-utils/blob/9ef54b472e42ba2c5479fbd86b8be2275724b064/scanelf.c#L428-L512
@tgross35 tgross35 force-pushed the check-wx-sections branch from ada5714 to 9f44c32 Compare May 30, 2025 02:44
@tgross35
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Thumb makes sense to skip as a bare metal target. Note sure about ppc64

@tgross35 tgross35 mentioned this pull request May 30, 2025
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tgross35 commented May 30, 2025 •
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Pinging some people who work on PowerPC in rust/llvm to : @daltenty @ecnelises @lei137

I'm trying a small C program on powerpc64, powerpc64le, and powerpc (32). For both powerpc32 and powerpc64le, as well as all other architectures I have tried, there is an empty section named .note.GNU-stack. As I understand it, if this section exists then the linker should not be writeable. However, it doesn't seem like this section is created on powerpc64 .

Is there some other way that a non-writeable stack is indicated, or is this not relevant on the powerpc64 arch?

Repro, note how there is no .note.GNU-stack section only for ppc64:

$ cat a.c
int moo() {
        return 10;
}

$ powerpc64-linux-gnu-gcc -c a.c -o ppc64.o
$ powerpc64le-linux-gnu-gcc -c a.c -o ppc64le.o
$ powerpc-linux-gnu-gcc -c a.c -o ppc32.o

$ powerpc64-linux-gnu-readelf -S ppc64.o
There are 12 section headers, starting at offset 0x268:

Section Headers:
  [Nr] Name              Type             Address           Offset
       Size              EntSize          Flags  Link  Info  Align
  [ 0]                   NULL             0000000000000000  00000000
       0000000000000000  0000000000000000           0     0     0
  [ 1] .text             PROGBITS         0000000000000000  00000040
       0000000000000030  0000000000000000  AX       0     0     4
  [ 2] .data             PROGBITS         0000000000000000  00000070
       0000000000000000  0000000000000000  WA       0     0     1
  [ 3] .bss              NOBITS           0000000000000000  00000070
       0000000000000000  0000000000000000  WA       0     0     1
  [ 4] .opd              PROGBITS         0000000000000000  00000070
       0000000000000018  0000000000000000  WA       0     0     8
  [ 5] .rela.opd         RELA             0000000000000000  000001d0
       0000000000000030  0000000000000018   I       9     4     8
  [ 6] .comment          PROGBITS         0000000000000000  00000088
       0000000000000026  0000000000000001  MS       0     0     1
  [ 7] .eh_frame         PROGBITS         0000000000000000  000000b0
       0000000000000034  0000000000000000   A       0     0     4
  [ 8] .rela.eh_frame    RELA             0000000000000000  00000200
       0000000000000018  0000000000000018   I       9     7     8
  [ 9] .symtab           SYMTAB           0000000000000000  000000e8
       00000000000000d8  0000000000000018          10     8     8
  [10] .strtab           STRTAB           0000000000000000  000001c0
       0000000000000009  0000000000000000           0     0     1
  [11] .shstrtab         STRTAB           0000000000000000  00000218
       000000000000004e  0000000000000000           0     0     1
Key to Flags:
  W (write), A (alloc), X (execute), M (merge), S (strings), I (info),
  L (link order), O (extra OS processing required), G (group), T (TLS),
  C (compressed), x (unknown), o (OS specific), E (exclude),
  D (mbind), p (processor specific)

$ powerpc64le-linux-gnu-readelf -S ppc64le.o
There are 11 section headers, starting at offset 0x228:

Section Headers:
  [Nr] Name              Type             Address           Offset
       Size              EntSize          Flags  Link  Info  Align
  [ 0]                   NULL             0000000000000000  00000000
       0000000000000000  0000000000000000           0     0     0
  [ 1] .text             PROGBITS         0000000000000000  00000040
       0000000000000030  0000000000000000  AX       0     0     4
  [ 2] .data             PROGBITS         0000000000000000  00000070
       0000000000000000  0000000000000000  WA       0     0     1
  [ 3] .bss              NOBITS           0000000000000000  00000070
       0000000000000000  0000000000000000  WA       0     0     1
  [ 4] .comment          PROGBITS         0000000000000000  00000070
       0000000000000026  0000000000000001  MS       0     0     1
  [ 5] .note.GNU-stack   PROGBITS         0000000000000000  00000096
       0000000000000000  0000000000000000           0     0     1
  [ 6] .eh_frame         PROGBITS         0000000000000000  00000098
       0000000000000034  0000000000000000   A       0     0     4
  [ 7] .rela.eh_frame    RELA             0000000000000000  000001b8
       0000000000000018  0000000000000018   I       8     6     8
  [ 8] .symtab           SYMTAB           0000000000000000  000000d0
       00000000000000d8  0000000000000018           9     8     8
  [ 9] .strtab           STRTAB           0000000000000000  000001a8
       0000000000000009  0000000000000000           0     0     1
  [10] .shstrtab         STRTAB           0000000000000000  000001d0
       0000000000000054  0000000000000000           0     0     1
Key to Flags:
  W (write), A (alloc), X (execute), M (merge), S (strings), I (info),
  L (link order), O (extra OS processing required), G (group), T (TLS),
  C (compressed), x (unknown), o (OS specific), E (exclude),
  D (mbind), p (processor specific)

$ powerpc-linux-gnu-readelf -S ppc32.o
There are 11 section headers, starting at offset 0x1b4:

Section Headers:
  [Nr] Name              Type            Addr     Off    Size   ES Flg Lk Inf Al
  [ 0]                   NULL            00000000 000000 000000 00      0   0  0
  [ 1] .text             PROGBITS        00000000 000034 000024 00  AX  0   0  4
  [ 2] .data             PROGBITS        00000000 000058 000000 00  WA  0   0  1
  [ 3] .bss              NOBITS          00000000 000058 000000 00  WA  0   0  1
  [ 4] .comment          PROGBITS        00000000 000058 000026 01  MS  0   0  1
  [ 5] .note.GNU-stack   PROGBITS        00000000 00007e 000000 00      0   0  1
  [ 6] .eh_frame         PROGBITS        00000000 000080 000038 00   A  0   0  4
  [ 7] .rela.eh_frame    RELA            00000000 000154 00000c 0c   I  8   6  4
  [ 8] .symtab           SYMTAB          00000000 0000b8 000090 10      9   8  4
  [ 9] .strtab           STRTAB          00000000 000148 000009 00      0   0  1
  [10] .shstrtab         STRTAB          00000000 000160 000054 00      0   0  1
Key to Flags:
  W (write), A (alloc), X (execute), M (merge), S (strings), I (info),
  L (link order), O (extra OS processing required), G (group), T (TLS),
  C (compressed), x (unknown), o (OS specific), E (exclude),
  D (mbind), v (VLE), p (processor specific)

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Clang seems to emit the section still, so I guess there could be a chance it is a bug with GCC.

$ clang --target=powerpc64-unknown-linux-gnu -c a.c -o ppc64-clang.o
$ powerpc64-linux-gnu-readelf -S ppc64-clang.o
There are 11 section headers, starting at offset 0x1e8:

Section Headers:
  [Nr] Name              Type             Address           Offset
       Size              EntSize          Flags  Link  Info  Align
  [ 0]                   NULL             0000000000000000  00000000
       0000000000000000  0000000000000000           0     0     0
  [ 1] .strtab           STRTAB           0000000000000000  00000180
       0000000000000065  0000000000000000           0     0     1
  [ 2] .text             PROGBITS         0000000000000000  00000040
       0000000000000014  0000000000000000  AX       0     0     4
  [ 3] .opd              PROGBITS         0000000000000000  00000058
       0000000000000018  0000000000000000  WA       0     0     8
  [ 4] .rela.opd         RELA             0000000000000000  00000138
       0000000000000030  0000000000000018   I      10     3     8
  [ 5] .comment          PROGBITS         0000000000000000  00000070
       0000000000000028  0000000000000001  MS       0     0     1
  [ 6] .note.GNU-stack   PROGBITS         0000000000000000  00000098
       0000000000000000  0000000000000000           0     0     1
  [ 7] .eh_frame         PROGBITS         0000000000000000  00000098
       0000000000000028  0000000000000000   A       0     0     8
  [ 8] .rela.eh_frame    RELA             0000000000000000  00000168
       0000000000000018  0000000000000018   I      10     7     8
  [ 9] .llvm_addrsig     LLVM_ADDRSIG     0000000000000000  00000180
       0000000000000000  0000000000000000   E      10     0     1
  [10] .symtab           SYMTAB           0000000000000000  000000c0
       0000000000000078  0000000000000018           1     3     8
Key to Flags:
  W (write), A (alloc), X (execute), M (merge), S (strings), I (info),
  L (link order), O (extra OS processing required), G (group), T (TLS),
  C (compressed), x (unknown), o (OS specific), E (exclude),
  D (mbind), p (processor specific)

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Parts of libcompiler_builtins are compiled as WX
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