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Bug/syscall return #276

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Merged
wipawel merged 2 commits into from
Jul 21, 2023
Merged

Bug/syscall return #276

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d63c9d0
README: mtools dependency
sktt Jul 15, 2023
32ae36d
usermode: syscall_handler set return with asm
sktt Jul 21, 2023
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2 changes: 1 addition & 1 deletion README.md
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Expand Up @@ -47,7 +47,7 @@ make boot

You may need to install the following (unless you already have it):
* GRUB2 bootloader tools - `grub2-common` package (e.g. `apt install grub2-common`)
* ISO generation tools - `xorriso` package (e.g. `apt install xorriso`)
* ISO generation tools - `xorriso` and `mtools` package (e.g. `apt install xorriso mtools`)

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@minipli-oss minipli-oss Jul 21, 2023

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mtools is needed for creating the ISO via grub-mkrescue? If so, you should mention that in the commit log, at least. However, it should already had been installed via the grub2-common package, as that depends on grub-common which "suggests" mtools. But yeah, it's not a strong dependency, so we may want to make it explicit.

#### Kernel image build (for example to be used with QEMU)

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9 changes: 7 additions & 2 deletions common/usermode.c
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Expand Up @@ -49,8 +49,13 @@ static inline void syscall_restore(void) {
}

static inline long syscall_return(long return_code) {
register long ret asm(STR(_ASM_AX)) = return_code;
return ret;
/* clang-format off */
asm volatile(
"mov %[ret], %%" STR(_ASM_AX) "\n"
::[ret] "r"(return_code)
);
/* clang-format on */
return return_code;
}
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@minipli-oss minipli-oss Jul 21, 2023

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WTF is this?! Why isn't this a simple return return_code?.... Oh, I see. syscall_handler(), what a bummer! It does the right thing only by chance. We'd better rewrite that think with a ASM thunk instead of this fragile register pinning code (gcc is free to reuse these registers, you know ;)).

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@wipawel wipawel Jul 21, 2023

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You are right of course, the current implementation of syscall handling is indeed quite fragile. I plan to improve it all at some point.


static inline void stack_switch(void) {
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