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loader.c uses AT_FLAGS to communicate literally

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Now the `ape - prog args...` form communicates to the loaded binary that
argv[0] is preserved. This is done via either editing the previous flags
passed in auxv, or adding a new AT_FLAGS value at the end if one was not
found.

I decided to always insert an AT_FLAGS entry, although another option is
to only insert one if literally is true. This would change the value for
`n` to `... + 2 * (!flags && literally) + ...` and add another branch to
the if statement. I guessed that the extra branch was not worth saving 2
words on the stack sometimes.
This commit is contained in:
Jōshin 2024-05-06 09:37:57 -07:00
parent 06d916b449
commit b77a3d5c67
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1 changed files with 17 additions and 5 deletions
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22
ape/loader.c
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@ -949,7 +949,7 @@ EXTERN_C __attribute__((__noreturn__)) void ApeLoader(long di, long *sp,
char arg0, literally;
unsigned long pagesz;
union ElfEhdrBuf *ebuf;
long *auxv, *ap, *endp, *sp2;
long *auxv, *flags, *ap, *endp, *sp2;
char *p, *pe, *exe, *prog, **argv, **envp;
(void)Printf;
@ -993,13 +993,15 @@ EXTERN_C __attribute__((__noreturn__)) void ApeLoader(long di, long *sp,
/* detect netbsd and find end of words */
pagesz = 0;
arg0 = 0;
flags = 0;
for (ap = auxv; ap[0]; ap += 2) {
if (ap[0] == AT_PAGESZ) {
pagesz = ap[1];
} else if (SupportsNetbsd() && !os && ap[0] == AT_EXECFN_NETBSD) {
os = NETBSD;
} else if (SupportsLinux() && ap[0] == AT_FLAGS) {
// TODO(mrdomino): maybe set/insert this when we are called as "ape -".
} else if (ap[0] == AT_FLAGS) {
// TODO(mrdomino): does anyone use auxv 8 for anything else?
flags = ap + 1;
arg0 = !!(ap[1] & AT_FLAGS_PRESERVE_ARGV0);
}
}
@ -1047,8 +1049,9 @@ EXTERN_C __attribute__((__noreturn__)) void ApeLoader(long di, long *sp,
/* create new bottom of stack for spawned program
system v abi aligns this on a 16-byte boundary
grows down the alloc by poking the guard pages */
n = (endp - sp + 1) * sizeof(long);
grows down the alloc by poking the guard pages
uses AT_FLAGS to say that argv[0] is preserved */
n = (endp - sp + 2 * !flags + 1) * sizeof(long);
sp2 = (long *)__builtin_alloca(n);
if ((long)sp2 & 15)
++sp2;
@ -1059,6 +1062,15 @@ EXTERN_C __attribute__((__noreturn__)) void ApeLoader(long di, long *sp,
argv = (char **)(sp2 + 1);
envp = (char **)(sp2 + 1 + argc + 1);
auxv = sp2 + (auxv - sp);
if (flags) {
flags = flags - sp + sp2;
flags[1] |= literally * AT_FLAGS_PRESERVE_ARGV0;
} else {
flags = (endp - 1) - sp + sp2;
flags[0] = AT_FLAGS;
flags[1] = literally * AT_FLAGS_PRESERVE_ARGV0;
/* end-of-auxv sentinel is zeroed by loop above */
}
sp = sp2;
/* allocate ephemeral memory for reading file */