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getrandom() backwards compatibility shim #10741

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Merged
LightBender merged 2 commits into from
Apr 10, 2025
Merged

getrandom() backwards compatibility shim #10741

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c132f38
Implement `version (linux_legacy_emulate_getrandom)`
0xEAB Apr 6, 2025
4a5098f
Fix style
0xEAB Apr 6, 2025
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4 changes: 4 additions & 0 deletions Makefile
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Original file line number Diff line number Diff line change
Expand Up @@ -173,6 +173,10 @@ ifdef NO_AUTODECODE
override DFLAGS += -version=NoAutodecodeStrings
endif

ifdef LINUX_LEGACY_EMULATE_GETRANDOM
override DFLAGS += -version=linux_legacy_emulate_getrandom
endif

UDFLAGS=-unittest -version=StdUnittest

LINKDL:=$(if $(findstring $(OS),linux),-L-ldl,)
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66 changes: 56 additions & 10 deletions std/random.d
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Expand Up @@ -1774,19 +1774,65 @@ else

version (linux)
{
// `getrandom()` was introduced in Linux 3.17.
version (linux_legacy_emulate_getrandom)
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@ibuclaw ibuclaw Apr 10, 2025

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import gcc.config;
static if (Have_Getrandom)

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@0xEAB 0xEAB Apr 12, 2025

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@ibuclaw Is this something you’d like me to incorporate?
Or was it more like a note to yourself?

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@ibuclaw ibuclaw Apr 12, 2025 •
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It would have been nice if this was an enum, as that can survive in the installed sources after the library has been built, and there's already other tests whose results get written to this module at configure time.

https://gcc.gnu.org/git/?p=gcc.git;a=blob;f=libphobos/libdruntime/gcc/config.d.in;h=e5fcd5073a96a2ffdee25c33348fc73bdddb6bbb;hb=HEAD

version (GNU)
{
    import gcc.config;
}
else version (linux_legacy_emulate_getrandom)
{
    enum Have_Getrandom = false;
}
else
{
    enum Have_Getrandom = true;
}

Have added this, though minor annoyance that it's its own thing rather than using the existing "features" module.

https://gcc.gnu.org/git/?p=gcc.git;a=blobdiff;f=libphobos/m4/druntime/os.m4;h=ef8ca434407149c176e5d93ab4012d53511048e3;hp=15cde3b04b8d47a66f83fa6ae56057359b66adac;hb=b905ce8caf04253e02e153d60d6ea8f99d300af6;hpb=882d3b319dbf50ae64080731a1398031c100b7c7

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@ibuclaw ibuclaw Apr 12, 2025

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Fortunately, this is a function, and not a template. Otherwise this shim would be a non-starter.

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@0xEAB 0xEAB Apr 12, 2025

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Anything urgent here?
Otherwise I’d rather wait to see this getting hopefully replaced by something like #10748 in the near future anyway.

{
/+
Emulates `getrandom()` for backwards compatibility
with outdated kernels and legacy libc versions.

`getrandom()` was added to the GNU C Library in v2.25.
+/
pragma(msg, "`getrandom()` emulation for legacy Linux targets is enabled.");
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@0xEAB 0xEAB Apr 6, 2025

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This is obviously a bit spammy. Maybe we should comment it out?


/+
On modern kernels (5.6+), `/dev/random` would behave more similar
to `getrandom()`.
However, this emulator was specifically written for systems older
than that. Hence, `/dev/urandom` is the CSPRNG of choice.

<https://web.archive.org/web/20200914181930/https://www.2uo.de/myths-about-urandom/>
+/
private static immutable _pathLinuxSystemCSPRNG = "/dev/urandom";

import core.sys.posix.sys.types : ssize_t;

/+
Linux `getrandom()` emulation built upon `/dev/urandom`.
Parameter `flags` is happily ignored.
+/
private ssize_t getrandom(
void* buf,
size_t buflen,
uint flags,
) @system nothrow @nogc
{
import core.stdc.stdio : fclose, fopen, fread;

// Shim for missing bindings in druntime
version (none)
import core.sys.linux.sys.random : getrandom;
auto blockDev = fopen(_pathLinuxSystemCSPRNG.ptr, "r");
if (blockDev is null)
assert(false, "System CSPRNG unavailable: `fopen(\"" ~ _pathLinuxSystemCSPRNG ~ "\")` failed.");
scope (exit) fclose(blockDev);

const bytesRead = fread(buf, 1, buflen, blockDev);
return bytesRead;
}
}
else
{
import core.sys.posix.sys.types : ssize_t;
extern extern(C) ssize_t getrandom(
void* buf,
size_t buflen,
uint flags,
) @system nothrow @nogc;
// `getrandom()` was introduced in Linux 3.17.

// Shim for missing bindings in druntime
version (none)
import core.sys.linux.sys.random : getrandom;
else
{
import core.sys.posix.sys.types : ssize_t;
private extern extern(C) ssize_t getrandom(
void* buf,
size_t buflen,
uint flags,
) @system nothrow @nogc;
}
}
}

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