<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/63579>63579</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            KCFI and Function Sanitizer cookies are unaligned
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            new issue
      </td>
    </tr>

    <tr>
      <th>Assignees</th>
      <td>
      </td>
    </tr>

    <tr>
      <th>Reporter</th>
      <td>
          statham-arm
      </td>
    </tr>
</table>

<pre>
    The following IR defines two functions, each of which compiles to a single 2-byte Thumb instruction, but prefixed with a 4-byte KCFI cookie (containing, in this case, a made-up example value).
```
target triple = "thumbv8a-arm-none-eabi"

define void @foo() !kcfi_type !1 {
entry:
  ret void
}
define void @bar() !kcfi_type !1 {
entry:
  ret void
}

!1 = !{i32 1234567890}
```

Compiled to an object file using either of
```
llc -filetype=obj kcfi.ll
clang --target=arm-none-eabi -mcpu=cortex-a53 -mthumb -c kcfi.ll
```
the resulting object file contains a 12-byte `.text` section, with no alignment padding, consisting of a 4-byte cookie, a 2-byte function, and the same again. This means that the two 32-bit cookies can't be aligned the same: one is aligned to a multiple of 4 bytes, and the other is misaligned. So a function call that loads and checks the cookie can only work if the CPU is configured to permit unaligned loads.

Adding `-mattr=+strict-align` to the `llc` command, or `-mno-unaligned-access` to the `clang` command, doesn't change the behavior. So it seems that there's no way to generate KCFI cookies that can be checked safely in no-unaligned-access mode.

As far as I can see, all of this works exactly the same for Function Sanitizer cookies.
</pre>
<img width="1px" height="1px" alt="" src="http://email.email.llvm.org/o/eJysVcFu47gS_Br60pAhUbZsHXSYxDAweJeHnex50aJaEicUaZCtON6vX5BSxvEge1sgUCCxq1jsKrYxBD1Yokbsn8T-tMGZR-ebwMgjThn6adO67ta8jAS9M8ZdtR3g-x_QUa8tBeCrg362irWzQchnIFQjuB6uo1YjKDddtIl1DhCCtoMhkFl7Y4KXcZ5a0DawnxM-wtuZ4eKp1-_UwVXzCAi7pf5_z-fvoJx71QRCHpWzjNpqO0SctsCjDqAwUHxHmLCjbL4AveN0MQRvaGYSst6K_CTyb6LK17_0yugHYmCvY60oTyCk5Kjw7YixD5l1ljLCVgspV4r0XDoBb053IHZ575yQRyFrELJ4Vb3-i2-XKLgoQByeFgxZ9jdRrgwAnjgRrLyH05fULfr_jHp9Jmg6bCEOT7qUUMhyt68Oxzq_1z62ank-L852yVkLrv1JiqHXhmCOPgNpHsmD678kMUZBFqvjEUR5cu1PiEfaGrMUKIN2gCxbjBHl6cEDyCZ1mUV5Us4zvWe4LyGbkl-QqUem350eCTyF2XBU-Vn3mqgACMWaUVHlW6Z3FlUOgX6lNAXTOkCjBzuRZbhg161RVM4GHRb2_h7fJblLNlf2j4uTPtoOorSAEwEOqO0WXmKiJ0IbgEfktB4vXCmzVvPKGDNvhTwwtLQIojuTKL-BswQ63JfiTZzi8WPSXQ87iGLCZxEuWRc312HFbeFHBH5IBoXGLKqMwy4kqBpJvYbEsN5TFaNhzQ2uzr-C7tPa8___jNzK2V4Ps180XchPmmG2HzoT7fZz5L6lFkdPsgmZvShPQj4F9lpxlmDRJnZpE1Hlxqj4QblpQtvF8zm_oK3Lfm2UoVIUwiM0pe83cOcoLI1WI9qBUm1LI75p51N7NEMgmu5ueRLyEGJSrniL9ANZ8vg4zNby2KqWlh5SBwF7Mrc4174QC5Pr6LE3AXr0gAG-J6ZAS9SMiRan0RgtCHEcKja3e9Z65-H84eoPtJr13-Q_tG03XVN2dVnjhpqiOh7q_aEu883Y9HVFRSd38khVR73sVNH1WB_bKt91ezpudCNzWeaVPBZlIfN6i3lVtEWlirpTRdHuxC6nCbXZGvM2bZ0fNjqEmZqq3B_qjcGWTEg_TFJaukJajMN3f9r4JmKydh6C2OVGBw53FtZsqEkdjqn898MBeronbjN704zMlxDHpzwLeR40j3O7VW4S8hz513_Zxbs4N4Q8J1VByHNS_U8AAAD__zTSXE4">