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Support read-only section for curve25519_x25519base_byte.S #247

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Support read-only section for curve25519_x25519base_byte.S #247

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9 changes: 6 additions & 3 deletions arm/curve25519/curve25519_x25519base_byte.S
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Original file line number Diff line number Diff line change
Expand Up @@ -594,8 +594,10 @@ S2N_BN_SYMBOL(curve25519_x25519base_byte):
ldr x0, [scalar]
ands xzr, x0, #8

adr x10, curve25519_x25519base_byte_edwards25519_0g
adr x11, curve25519_x25519base_byte_edwards25519_8g
adrp x10, curve25519_x25519base_byte_edwards25519_0g
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@ctz ctz Jun 27, 2025

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AIUI page-level relocations for mach-o require @PAGE on the end of this, and @PAGEOFF on the end of the add operand (which also needs to lose the ELF-specific :lo12: relocation designation.)

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@ctz ctz Jun 27, 2025

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The other possibility is just requiring these to be page-aligned, which means we can lose the add adjustments and only need to support one kind of relocation specifier. That's what I did over here (macro def for reference).

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@jargh jargh Jun 27, 2025

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Yes, indeed Mach-O is a bit different. I think, after a bit of testing, that the following works with both Linux generating ELF and Mac OS generating Mach-O. (I just use ..._data since I think it will simplify the proof if we just have a single relocated symbol for the start of the data.) I'm confident we can prove this since I've prototyped something similar. But I suspect it needs a bit more refinement for other platforms. I'm hoping that if we delete the linux part of the ifdef it would work on OpenBSD and indeed other BSDs - does that look reasonable? (tab is just cpp'd to a register).

#if defined(__linux__) && defined(__ELF__)
        adrp    tab, curve25519_x25519base_byte_data
        add     tab, tab, :lo12:curve25519_x25519base_byte_data
#else
        adrp    tab, curve25519_x25519base_byte_data@PAGE
        add     tab, tab, curve25519_x25519base_byte_data@PAGEOFF
#endif

add x10, x10, :lo12:curve25519_x25519base_byte_edwards25519_0g
adrp x11, curve25519_x25519base_byte_edwards25519_8g
add x11, x11, :lo12:curve25519_x25519base_byte_edwards25519_8g
ldp x0, x1, [x10]
ldp x2, x3, [x11]
csel x0, x0, x2, eq
Expand Down Expand Up @@ -651,7 +653,8 @@ S2N_BN_SYMBOL(curve25519_x25519base_byte):
// l >= 9 case cannot arise on the last iteration.

mov i, 4
adr tab, curve25519_x25519base_byte_edwards25519_gtable
adrp tab, curve25519_x25519base_byte_edwards25519_gtable
add tab, tab, :lo12:curve25519_x25519base_byte_edwards25519_gtable
mov bias, xzr

// Start of the main loop, repeated 63 times for i = 4, 8, ..., 252
Expand Down