| Issue |
126629
|
| Summary |
`#embed` doesn't support reading from devices
|
| Labels |
|
| Assignees |
|
| Reporter |
zygoloid
|
Clang's `#embed` implementation doesn't behave properly when applied to a device:
```c++
int per_build_entropy[] = {
#embed "/dev/random" limit(1024)
};
```
Instead it treats devices as being empty -- in the above case, this leads to the array being initialized with zeroes! This is probably because we're using the `st_size` field produced by `stat`, but that only lists the size of the file if applied to a regular file:
> `st_size`
> > This field gives the size of the file (if it is a regular file or a symbolic link) in bytes. The size of a symbolic link is the length of the pathname it contains, without a terminating null byte.
While we try to [detect this case](https://github.com/llvm/llvm-project/blob/3c6d1dd362009e0aebd91c5197d40f8ce59fcff4/llvm/lib/Support/MemoryBuffer.cpp#L493) and use stream IO instead of mmap, we only do so when we weren't given a `MapSize`, which in this case we presumably will be due to the `limit(1024)`. And the code that handles the case where we have no `MapSize` does *not* handle the case where we're opening a device rather than a regular file. Perhaps `shouldUseMmap` should return `false` unless it's operating on a regular file?
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