Doesn't allowing a user mode program to access kernel space memory and execute the IN and OUT instructions...
When the CPU is in user mode, the CPU can't execute privileged instructions and can't access kernel space memory.
And when the CPU is in kernel mode, the CPU can execute all instructions and can access all memory.
Now in Linux, a user mode program can access all memory (using /dev/mem) and can execute the two privileged instructions IN and OUT (using iopl() I think).
So a user mode program in Linux can do most things (I think most things) that can be done in kernel mode.
Doesn't allowing a user mode program to have all this power defeats the purpose of having CPU modes?
linux
edited 4 hours ago
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When the CPU is in user mode, the CPU can't execute privileged instructions and can't access kernel space memory.
And when the CPU is in kernel mode, the CPU can execute all instructions and can access all memory.
Now in Linux, a user mode program can access all memory (using /dev/mem) and can execute the two privileged instructions IN and OUT (using iopl() I think).
So a user mode program in Linux can do most things (I think most things) that can be done in kernel mode.
Doesn't allowing a user mode program to have all this power defeats the purpose of having CPU modes?
linux
edited 4 hours ago
ilkkachu
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asked 5 hours ago
user341099user341099
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When the CPU is in user mode, the CPU can't execute privileged instructions and can't access kernel space memory.
And when the CPU is in kernel mode, the CPU can execute all instructions and can access all memory.
Now in Linux, a user mode program can access all memory (using /dev/mem) and can execute the two privileged instructions IN and OUT (using iopl() I think).
So a user mode program in Linux can do most things (I think most things) that can be done in kernel mode.
Doesn't allowing a user mode program to have all this power defeats the purpose of having CPU modes?
linux
edited 4 hours ago
ilkkachu
60.7k1098172
asked 5 hours ago
user341099user341099
111
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When the CPU is in user mode, the CPU can't execute privileged instructions and can't access kernel space memory.
And when the CPU is in kernel mode, the CPU can execute all instructions and can access all memory.
Now in Linux, a user mode program can access all memory (using /dev/mem) and can execute the two privileged instructions IN and OUT (using iopl() I think).
So a user mode program in Linux can do most things (I think most things) that can be done in kernel mode.
Doesn't allowing a user mode program to have all this power defeats the purpose of having CPU modes?
linux
linux
edited 4 hours ago
ilkkachu
60.7k1098172
asked 5 hours ago
user341099user341099
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edited 4 hours ago
ilkkachu
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edited 4 hours ago
ilkkachu
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edited 4 hours ago
ilkkachu
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edited 4 hours ago
ilkkachu
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60.7k1098172
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So a user mode program in Linux can do most things (I think most things) that can be done in kernel mode.
Well, not all use mode programs can, only those with the appropriate privileges. And that's determined by the kernel.
/dev/mem is protected by the usual filesystem access permissions, and the CAP_SYS_RAWIO capability. iopl() and ioperm() are also restricted through the same capability.
/dev/mem can also be compiled out of the kernel altogether (CONFIG_DEVMEM).
Doesn't allowing a user mode program to have all this power defeats the purpose of having CPU modes?
Well, maybe. It depends on what you want privileged user-space processes to be able to do. User-space processes can also trash the whole hard drive if they have access to /dev/sda (or equivalent), even though that defeats the purpose of having a filesystem driver to handle storage access.
(Then there's also the fact that iopl() works by utilizing the CPU privilege modes on i386, so it can't well be said to defeat their purpose.)
edited 4 hours ago
Stephen Kitt
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answered 4 hours ago
ilkkachuilkkachu
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So a user mode program in Linux can do most things (I think most things) that can be done in kernel mode.
Well, not all use mode programs can, only those with the appropriate privileges. And that's determined by the kernel.
/dev/mem is protected by the usual filesystem access permissions, and the CAP_SYS_RAWIO capability. iopl() and ioperm() are also restricted through the same capability.
/dev/mem can also be compiled out of the kernel altogether (CONFIG_DEVMEM).
Doesn't allowing a user mode program to have all this power defeats the purpose of having CPU modes?
Well, maybe. It depends on what you want privileged user-space processes to be able to do. User-space processes can also trash the whole hard drive if they have access to /dev/sda (or equivalent), even though that defeats the purpose of having a filesystem driver to handle storage access.
(Then there's also the fact that iopl() works by utilizing the CPU privilege modes on i386, so it can't well be said to defeat their purpose.)
edited 4 hours ago
Stephen Kitt
175k24400477
answered 4 hours ago
ilkkachuilkkachu
60.7k1098172
add a comment |
So a user mode program in Linux can do most things (I think most things) that can be done in kernel mode.
Well, not all use mode programs can, only those with the appropriate privileges. And that's determined by the kernel.
/dev/mem is protected by the usual filesystem access permissions, and the CAP_SYS_RAWIO capability. iopl() and ioperm() are also restricted through the same capability.
/dev/mem can also be compiled out of the kernel altogether (CONFIG_DEVMEM).
Doesn't allowing a user mode program to have all this power defeats the purpose of having CPU modes?
Well, maybe. It depends on what you want privileged user-space processes to be able to do. User-space processes can also trash the whole hard drive if they have access to /dev/sda (or equivalent), even though that defeats the purpose of having a filesystem driver to handle storage access.
(Then there's also the fact that iopl() works by utilizing the CPU privilege modes on i386, so it can't well be said to defeat their purpose.)
edited 4 hours ago
Stephen Kitt
175k24400477
answered 4 hours ago
ilkkachuilkkachu
60.7k1098172
add a comment |
So a user mode program in Linux can do most things (I think most things) that can be done in kernel mode.
Well, not all use mode programs can, only those with the appropriate privileges. And that's determined by the kernel.
/dev/mem is protected by the usual filesystem access permissions, and the CAP_SYS_RAWIO capability. iopl() and ioperm() are also restricted through the same capability.
/dev/mem can also be compiled out of the kernel altogether (CONFIG_DEVMEM).
Doesn't allowing a user mode program to have all this power defeats the purpose of having CPU modes?
Well, maybe. It depends on what you want privileged user-space processes to be able to do. User-space processes can also trash the whole hard drive if they have access to /dev/sda (or equivalent), even though that defeats the purpose of having a filesystem driver to handle storage access.
(Then there's also the fact that iopl() works by utilizing the CPU privilege modes on i386, so it can't well be said to defeat their purpose.)
edited 4 hours ago
Stephen Kitt
175k24400477
answered 4 hours ago
ilkkachuilkkachu
60.7k1098172
So a user mode program in Linux can do most things (I think most things) that can be done in kernel mode.
Well, not all use mode programs can, only those with the appropriate privileges. And that's determined by the kernel.
/dev/mem is protected by the usual filesystem access permissions, and the CAP_SYS_RAWIO capability. iopl() and ioperm() are also restricted through the same capability.
/dev/mem can also be compiled out of the kernel altogether (CONFIG_DEVMEM).
Doesn't allowing a user mode program to have all this power defeats the purpose of having CPU modes?
Well, maybe. It depends on what you want privileged user-space processes to be able to do. User-space processes can also trash the whole hard drive if they have access to /dev/sda (or equivalent), even though that defeats the purpose of having a filesystem driver to handle storage access.
(Then there's also the fact that iopl() works by utilizing the CPU privilege modes on i386, so it can't well be said to defeat their purpose.)
edited 4 hours ago
Stephen Kitt
175k24400477
answered 4 hours ago
ilkkachuilkkachu
60.7k1098172
edited 4 hours ago
Stephen Kitt
175k24400477
edited 4 hours ago
Stephen Kitt
175k24400477
edited 4 hours ago
Stephen Kitt
175k24400477
175k24400477
answered 4 hours ago
ilkkachuilkkachu
60.7k1098172
answered 4 hours ago
ilkkachuilkkachu
60.7k1098172
answered 4 hours ago
ilkkachuilkkachu
60.7k1098172
60.7k1098172
add a comment |
add a comment |
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